upgrading to lithium batteries and dealing with charging from the alternator

I am upgrading my RV which is a 2009 2551 to 2 100ah Lithium batteries connected in parallel.  I have a Xantrex Freedom XC 2000 inverter/charger that can charge lithium and I have a Go Power solar PWM controller with a 170watt panel that will also work with Lithium. Currently when I am running my engine, my inverter/charger will show that the alternator is charging the house batteries.
I have ordered a Victron DC-to-DC 12/12/-30amp charger for the conversion as I had heard about concerns regarding the alternator charging lithium batteries especially if the battery charge level is extremely low.  In any case I bought the charger.  My question is when I install the charger, how do I handle the current charging system so that it is only charging through the new DC To DC charger setup when the engine is running?  I heard about the solenoid in the engine compartment under the coolant overflow container and have taken photos.  The problem is, I do not know where the wires are going except for one that goes to the chassis battery.  The other wires I have now idea where they go and what one or ones I need to disconnect if any after I install the new system.  Like I mentioned I know that one of the two larger gauge red wires go to the chassis battery and the other looks like it is connecting to a fuse and two other wires on the fuse.  There are then two smaller posts at the base.  One of the smaller posts has a black wire that I think may go to ground and on the other post are three wires.  Two red wires and one that looks like a black wire.  I have no idea where they go or their use.  Maybe one red wire that looks like it goes through the firewall is for the white switch under the dash on the driver’s side used in case you need to jump your chassis battery using your house battery?  The other red wire and Black wire I have no clue.
I am hoping someone can explain how that solenoid works along with how it is wired up (a diagram would be great), and when I have installed the DC-to-DC charger, what do I need to do if anything with regards to the solenoid?
I am concerned about both systems trying to charge the batteries.
Any input would be greatly appreciated.

I have never posted before, so I hope this goes through okay.

Thanks for any help,
Proud Phoenix Cruiser owner of a 2009 2551

Thank you CalCruiser for your diagram.  I would loved to know what program you used to make the diagram as I am currently trying do do one the hard way by hand.

Thank you Mike for your input.  I went back to my RV to try and trace where some more of the wires went and this is what I came up with.

from the 3 small wires that came from the one small post, one of the red wires went up and through the firewall into the area under the drivers side console area and I could not figure where it went after it came out from the fire wall.  the other small red wire from the same post, went to a group of wires which went into a socket that plugged into another socket and that group of wires went up and ran along a channel at the firewall, but I am not sure were it went from there.  What I found interesting was the other wires that came out of where the red wire went into went for about three inches, was all taped up and was a dead end (was not attached to anything).  I will try and post some pictures of that.  The one black wire that came from the same post as the two red wires wrapped around and up through the firewall and out to under the console area where it looks like it is attached to the White momentary switch.  Now from the fuse/breaker, the black wire that comes off the post goes up to the firewall same place the other black wire went comes out the other side and is connected to the other end of the momentary switch.  Finally the red wire coming off the fuse/breaker goes up to and through the firewall together with the other wires mentioned and out the other side.  I was unable to determine where this wire goes.  Looks like it went under the carpet on the drivers side.

So if there is anyone that can help complete where all these wires go, I would greatly appreciate it as I can then complete my diagram and have a better understanding of how all this works and how the installation of my DC to DC charger will affect this system.

Thanks again for the inputs.

OK--let me take the wires directly from your post one at a time and address them that way.

"I went back to my RV to try and trace where some more of the wires went and this is what I came up with:
from the 3 small wires that came from the one small post, one of the red wires went up and through the firewall into the area under the drivers side console area and I could not figure where it went after it came out from the fire wall."  This is the wire that I said is providing a "key-on" feed from that solenoid terminal TO some other circuit that needs voltage only when the key switched on.  I'm not sure exactly what circuit that is--it would not be a OEM-Ford chassis item, but something that Phoenix installed when they built the coach.  It is feeding voltage FROM that solenoid terminal to something.

"the other small red wire from the same post, went to a group of wires which went into a socket that plugged into another socket and that group of wires went up and ran along a channel at the firewall, but I am not sure were it went from there.  What I found interesting was the other wires that came out of where the red wire went into went for about three inches, was all taped up and was a dead end (was not attached to anything).  I will try and post some pictures of that."  This particular red wire is providing the 12-volt signal to energize the solenoid when the key is switched on.  It is feeding voltage TO that small solenoid terminal from a Ford wiring plug that Ford furnishes in the wiring to provide power for needed add-on circuits.  There are two of those plugs, one is battery (full-time) 12-volts, and one is key-switched 12-volts; that particular one is the key-switched 12 volts.  For the purpose of a wiring diagram, it essentially feeds from the key switch.

The one black wire that came from the same post as the two red wires wrapped around and up through the firewall and out to under the console area where it looks like it is attached to the White momentary switch.  This black wire will feed 12-volts  from the "white switch" TO the solenoid terminal when the "white switch" is pushed.  It is how the solenoid can be energized when you turn the key on but the Ford chassis battery is too dead to do it (the white switch gets its 12-volts from the house batteries). 

Now from the fuse/breaker, the black wire that comes off the post goes up to the firewall same place the other black wire went comes out the other side and is connected to the other end of the momentary switch.  This is the 12-volt feed TO the white switch from the house batteries--that 40-amp self-resetting circuit breaker terminal is connected to the house batteries.

Finally the red wire coming off the fuse/breaker goes up to and through the firewall together with the other wires mentioned and out the other side.  I was unable to determine where this wire goes.  Looks like it went under the carpet on the drivers side.  This is the wire that goes to the positive terminal on the house batteries.  It provides the alternator charge current to the house batteries when the solenoid is energized.  Where it actually connects could vary--on my 2019, it actually goes to the positive battery terminal of my house batteries, but it could tie anywhere into the main positive circuit and work the same.  Electrically you could show it just going to the house battery positive terminal.

The + side of  your DC-DC converter needs to be connected in series between the alternator and  house batteries. Does Victron recommend locating it under the hood? Looking at your pictures, an easy way might be to cut the heavy gauge red wire that goes from the  battery isolating relay to the 40A self resetting breaker. Then connect the relay side to IN and the breaker side  to OUT.

If  Victron didn’t include a length of black wire and a ring or U terminal for the GND connection, the instructions should specify what gauge wire to get. Chassis ground can be any unpainted bolt under the hood or on the frame, where the ground wire can’t touch the hot engine or exhaust system.

So if there is anyone that can help complete where all these wires go, I would greatly appreciate it as I can then complete my diagram and have a better understanding of how all this works

Year to year there are minor differences but this is the basic PC power schematic.