Hello Phil
Don't let Donc13 scare you off, the price of the Houghton A/C's has dropped and became available through RecPro.com as Jim Godfrey pointed out in a post previous to this one (Any issue running Houghton AC while driving?). The install is very easy, actually the mounting of the Houghton A/C's in my opinion is much more robust and offers a better seal to the roof than the Dometic units.
Donc13 mentions that the thermostat for the heater needs to be rewired that is correct, but it is a very easy process if you just replace the Dometic thermostat with a standard mechanical one. The Houghton A/C units are self contained and don't require a external thermostat to operate and also come with a remote control.
My PC has 2 of them one was installed by Advanced RV and the other by me. Advanced RV had installed a control box that allowed the heater to use the original Dometic thermostat to operate just the heater. I removed this control box because Advanced RV installed it inside of the Houghton A/C blocking the air flow from the forward vents. So I just installed a mechanical thermostat for the heater for $15 and it works excellent plus it makes troubleshooting very simple for the heater. I used a drywall lift to get my A/C up to the roof, and installed it in the vent opening just forward of the bathroom. The first Houghton A/C is installed where the original Dometic was.
I just looked at the price for the Houghton 13.5K heat pump A/C from RecPro and they are selling it for $1449.95. or the straight cool for $1049.95. These A/C's are quiet, nothing like the loud wind tunnel of the Dometic units. Plus these A/C's will fit the same opening with no modification as the Dometic unit. FYI my Houghton A/C's are straight cool and cost $2500 without installation when they were purchased. So Donc13's $4000.00 price would be correct if you let Advanced RV do your install. I however would say that the hardest part is just getting the A/C on the roof, Houghton has made these A/C's extremely easy to install and if you have the ability to get the old one down and new one up on your roof your good to go.
Here are a couple pictures of my A/C's on the roof and how the second A/C operates as I don't have a 50amp service. If you have any questions or want some more pictures feel free to just ask. I believe that the upgrade to these Houghton A/C's is well worth the time and money.
Very Respectfully
Lance