Just a report on the Dometic heat pump AC unit that is standard factory issue now on the FC series, including my 20'.
I rarely use AC other than when it may be hot at home, and I have power and want to work in the trailer. However as the unit is new, this year I ran it for about 2 weeks to get some hours on it, in case it had any early failure issues which would be covered by warrantee. It seemed to cool well although somewhat noisy. No worse though than any other RV AC unit I have had over the years. I hate the complex thermostat, although I have become used to how to operate it now. But you can set it so the fan cycles with the compressor, so the fan is not roaring inside and out all the time. I like that.
I have just returned from a trip to Glacier Park area. I have friends who own a cabin just outside the park, and I can stay there, and have power. It was wet, cold, rainy and even a little snow. I thought I would try the heat pump part of the AC unit. To my great surprise, it worked pretty well. It dropped to 35 a couple of mornings, and the high temp was in the mid 40's while I was there. The heat pump continued to pump nice warm air even when it was 35 out (no heat strip on these units either, so it had not switched to all electric resistance). I had it set so the fan cycled off and on when it needed heat, and that also worked well. No cold drafts when the compressor was off but the fan running, just warm heat and air when the thermostat called for heat. Occasionally the unit went into defrost mode, where I could hear the compressor running, but the inside fan was shut off, so no cold air was cycled into the trailer.
I also could step out of the shower, push the outlet to blow warm air straight down on my head and dry my hair quickly, as well as the top half of my body. Kinda nice.
After reading all the comments on how poor the heat pump features were on older RV AC units, I was pleasantly surprised to find how well this new 2014 one works. I had never had a heat pump AC unit in the past.
So, the new ones do work into the mid 30's at least, have defrost capability, and produce good amount of heat. Still noisy, but I can live with that.