The best thing to do is to call the factory and ask them.
On our first PC, the 2350, it had the three drain lines in the passengers side outside compartment just above the drawer in the right hand wall. The current pictures of the 2910 show a cover in this position. Our current 2552 has the fresh water tank under the passengers side bed and the bed board has a cutout that allows me to reach in and open the valve. The hot and cold drains are under the kitchen sink on the floor. You have to remove the bottom drawer to get to them. Before you ask, to remove a drawer, any of the drawers, you pull the drawer open until it stops, then on each side, at the end of the slider, you will find a pointed plastic part that looks like an arrow. On the right side you push down on the plastic arrow, at the same time on the left you pull up on the arrow and gently pull the drawer forward. I find that I don't have to fully remove the drawer to reach in and turn the valves. I marked one side of the drawer with an arrow to help me remember which side goes which way. When done you just push the drawer back into the closed position.
Kermit told me that to speed up the draining of the fresh water tank, I should open the valve and turn on the water pump. I don't see how this will help on the new 2552 as the drain is separate from the line to the pump. One can always just pump the water into the gray tank via the sinks and then dump the gray, as has been suggested above.
On both of my PCs I found that the water was hardly coming out of the drain. So I took an air hose and put it up the drain hose and blew the line out. When they drill into the tank to put the hoses in sometimes a little of the tank plastic stays in the tank. This can then stop up the drain.
In 2011 Phoenix did put an outside spigot to drain the tank, but there where a lot of complains by the owners so they stopped doing this. I think they call this a running modification. They started late 2010 and and early 2012 I think.
While you have access under the sink, you might want to check the water heater bypass switches. The two outside lines should be open, valve lined up with the line, and the crossover line (bypass) should be closed, valve 90 degrees to the line. When winterizing you reverse these to bypass the water heater and save on the RV antifreeze.
I hope this helps.
