Not sure about sizing for your trailer, but my A126 has the Dometic CoolCat heat pump. It works great and from what I could see on the installation instructions in the manual it could be setup for ducted systems (in the A-series units it just has a single set of 3 outputs under the couch/dinette with return under those vents).
The CoolCat system however only works down to about 40 degrees F according to the manual. We have had success with ours working down to 36F before the compressor would shut down (manual states 40F +/- 2F).
We love the CoolCat and have only used our furnace for about an hour so far. We camped this past weekend and hit 34F just before sunrise. We turned on the gas furnace for a bit to warm up while we had coffee and waited for the sun to start warming things up.
2011 FR Rockwood Premiere A126 Hard Side
TV=2011 Jeep Wrangler Sahara 2 Dr
Reese 66065 WDH, Curt WDH Shank #17120
Prodigy P2 Brake Controller, Mopar 7-pin Harness
B&M Transmission Cooler (#70268)