Maybe an obvious question is: is your FW wired at 50 or 30 amp? If 30, then you may be limited to running one at a time, unless you run a separate power source to the second unit. Just before I sold my 2000 Hurricane I had to replace the roof AC and the new unit was a Coleman 13500 which did a good job. I think Coleman (or whoever really makes them) are a good unit.
My new 2016 Bay Star Sport had a Dometic 15k heat pump and we chose to use the heat side the other week on our maiden voyage. It dipped into the high 30's one night and it worked just fine. I'm not sure how efficient a roof mounted heat pump is considering in our case the return ducts are right next to the output vents.
Obviously we have not had it long enough or under really hot conditions to test the cooling in summer conditions.