I used 1" Styrofoam sheets cut to fit between studs in the walls and ceiling, used residential fiberglass in the curved section between walls and ceiling. I have a 6,500 BTU window unit, and live in NW Georgia. It's enough...but of course you can always do better. We have a Styrofoam manufacturer near us, so I just purchased enough 4 x 8 sheets at a bargain to get the job done.
Build details here:
http://www.tnttt.com/viewtopic.php?f=42&t=48345Air Conditioning here:
http://www.tnttt.com/viewtopic.php?f=42&t=49438#p956281If I were to do it again, I'd probably use a better foam, thicker, and definitely bridge the ribs with something so the interior screws wouldn't transfer heat or cold to the interior. I've also seen some fasteners that have plastic caps/covers made onto them.
We spent a night in the mid 20s boon docking with a little propane heat to assist, and cracked open roof vent and cracked open windows. Had a fair amount of condensation on the interior metal fasteners, more than likely due to breathing and the moisture content of propane combustion. Another surprise that night was how cold plywood floor got! Wind steady blowing under the trailer really created a heat sink situation on the floor. We later simply bought one of those commercial vinyl/carpet mats just about the right size to cover the entire trailer floor except a couple inches on each side, and it really made a difference. BTW, this is the only time we ever had a sweating issue, and it really wasn't a problem, just an interesting phenomenon.
Last Fourth of July near Calhoun, GA, we camped with friends who brought tents. We purposely didn't bring the A/C unit as a courtesy, and ran just the Fantastic Vent fan. It was ok, but I could tell the roof insulation was struggling during the heat of the day, we were also in direct sun all day. I have two E-track brackets attached directly to the roof studs, and they were warm to the touch due to heat transfer from the roof through the fasteners. Lots of folks coat the roof with Kool Seal, I've considered that, but when we install our modular A/C "closet" it has no problem keeping the interior comfortable even on the hottest days.
Back to winter, if we camp with power, a simple ceramic bathroom style space heater works wonders, even on the coldest nights. The next off-grid step for me would be one of the Dickinson propane marine heaters some have installed on here. Pricey, but really a great unit and solution for most of us.
So don't sweat it, but do insulate with something! I was ready to get it done and get going. The few things I've learned and would "do over" really haven't been enough of an issue to go back and fix it.