The Original Dometic Duo-Therm died after 23 years. (No lifetime warranty)! I thought it would be best to use a new Dometic Brisk II so I could use my digital wall mounted thermostat. Before you make the same mistakes I made let me tell you the problems:
1. The original used a bi-metallic mechanical thermostat on the walls, did not need any power as it was just changing a ground;
2. The original connected the ac power to the rear of the roof top unit with the wires in the return air intake area, but tucked up away from the intake filters.
3. The new ones use a digital thermostat and for that there must be 12 vdc power to the rooftop unit.
4. Because using a wall thermostat in new one is related to ducted air systems you have to install a control box and it goes in the return air inlet on top of the filter.
Ok, so after removing the old unit I discovered you can't use the same ceiling ADB (air distribution box) because the Duo-Therm uses three mounting bolts in a triangle but the new on uses four in a square pattern. Then of course the electrical service - both 110 ac and a 12vdc for the thermostat have to be run to the front of the air opening. Well you can't do that unless you tear out the ceiling like Nick did and run or alter your electrical circuits! They also want 15" of wire available and you might have 12". That is enough, however.
You can solve the dc voltage problem by opening bedroom drawer door where the tv is mounted and remove the wall outlet that has the old dc plug in for the ancient tv that will not work! Then drill a hole in the cabinet wall through the pocket door opening (being careful to have the door closed so you won't hit the door with the drill) and continue through the closet to the forward wall. There are four screws covered by wood filler that hold the thermostat box inside the little door and once removed the little box will easily push in and turn. Now you have access to the old 8 wire thermostat grey cable. You need the black, red, blue and yellow wires for the wall thermostat (did I mention you have to buy a new thermostat?) but you then have four extra, orange, grey, purple and brown. I tied the orange and grey together to become the 12v Red and the purple and brown to become the 12v Black. Connect with a soldering iron and heat shrink and then replace all the stuff. At the ceiling unit fabricate the end plug as instructed so the new telephone plug is aligned as shown in the booklet. Test the thermostat for lighting up before closing anything permanently. Then connect the same orange/grey (+) and brown/purple(-) and you are done. You may not need the wires tied together as I did but since they are 26 ga I decided 2, 26 ga wires were better than one.
Now of course the control box has to be stuffed into the return air cavity before you can tighten the ceilng pattern plate and squash down the roof top unit. They leave closed cell foam pads on the top unitl that look like padding for shipment but I discovered that they work as indicators so that when you tighten the long bolts they will contact the roof and prevent overtightening the bolts. This is the only good idea in the box!
The one remaining problem is the ceilng unit has no connection to the metal plate that holds the top,unit to the coach. They tell you to center the ADB so the return air hole is completely covering the intake air return so as not to suck cold air into the return when running, and screw the ADB to the ceiling. That's stupid! Screws in an upholstered ceilng will not hold and the compression necessary to squeeze the ADB up tightly either strips out the screws or can crack the plastic ADB. Better idea is mount rivet nuts in four of the extra holes they have in the steel pan and just screw through the ADB to the metal plate. (The real solution is to use a compound mounting bolt that has a nut holding the roof top unit with enough left over thread to extend through th ADB and then us a decorative acorn nut to mount the ADB.
If I was smarter or had this experience before I would have the old unit repaired with a new compressor, fan and even condenser, and outside shroud before I would ever buy another Dometic Brisk II. By the time you buy the unit ($600), the thermostat ($94), the digital "brain", ($148) and the new ADB ($190) you can certainly fully replace every moving part of your Duo-Therm.
I have not looked around for new stuff as I am still working to remount the ceiling panels that got wet from our recent rains.
Good luck!

I went through the same thing; the Penguin II ate 3 on-shroud thermostats and 3 wall thermostats before failing completely after 7 years. Changing the 'stat location required a different shroud, leaving exposed the screw holes in the headliner.

I had the Atwood installed, and it's much quieter and more powerful than the Dometic. The heat pump is good to about 39°. It requires only the 120VAC wiring. It also has a remote. The only drawbacks are that the Atwood's louvers adjust only up and down and the fan runs all the time (but it's very unobtrusive).

When I removed my rear AC and replaced it with a 12V Fantastic Vent I had the same problem. Fortunately there was a ceiling light feed near that I could fish a wire for 12V.

If not, you could always buy a small 12V convertor, attach to the 110AC to get a small local source of 12VDC for the controls, etc.

I recently ordered a new Atwood AC unit. I will expect much better performance and more quiet than the old one.

I asked Kevin about the 12v issue and he advised running a wire from the hot side of the front ceiling vent fan switch under the headliner. Tried that and it would not work so I called and Dometic insisted the 12v wire had to be a dedicated circuit. Imagine running a new circuit from the main panel in the front compartment through all the cables to the ceiling. Only after I figured out the old tv circuit was I able to get power to work.