All right first handle knowledge and usuage so take it for what its worth. I have a Custom Mirage that we had built with an Onan generator in it and a RVP Coleman-Mach Roughneck Rooftop AC/Heater. The AC works amazing, keeping in mind you have to have a generator strong enough to push it 2500KW or larger to be safe. Cools the trailer down like you wouldnt believe. We have had it in 100+ temps and it cools it to 60 no problem. The heater, is alittle different story, because it is realistically only air blowing across a heat strip you cant ask for a whole lot. It will take the chill off, but I cant tell you it is an answer to cold days. They rate them at something like 5000BTU equivalent (but since it is just heated air, it is localized and takes along time to equalize throughout the trailer - hours) There may be some better units out there but they are all typically the same, with enclosed trailers you need something really durable and hence the reason we went with the Roughneck because they take jarring and shifting really well. Hope this helps
Oh and insulated trailer is a must, if not forget about doing either