Don't make it not open!!!! You _have_ to make this thing open like you were planning. The cool factor is just too dang high.
By FAR the easiest thing is some air springs. You wouldn't need compressor/air lines/valves/etc.. The most expensive part of my pnumatic lifts was the valve manifold. It was a PITA to build too.
Yup. The screen Door closer pnumatics would work ok. They don't have _alot_ of lift, and they're not all that "safe" -- but they'd help. Or the similar and equally "unsafe" PVC version -- they wouldn't really be unsafe in your application, if the lifts were inside and depressurized when people are in there.
Commercial version:
http://www.surpluscenter.com/item.asp?UID=2009051411385727&item=4-1112&catname=air
A rodless pnumatic cylinder could work well. The ones on my trailer have about 80lbs of lift each. About 48" of movement in a 50" rod. You have to find them surplus 'cause they're very not-cheap new.
Simple small (surplus) hydraulics would work great too.
I built a prototype of a simple (read cheap) jackscrew setup that would work for your trailer.
1. Surplus/Junkyard power window motor connected to a chunk of 3/8" all thread. (Connected via a stout piece of fuel line hose for a bit of bendibility.)
2. The all thread runs up inside of a length of 3/4" PVC -- this is just to keep it clean, as I felt for 40-50lbs of lift the allthread would need to be well greased.
3. Press fit a 3/8" nut into some 1/2" EMT, and it fits inside the PVC, threaded onto the allthread. The other end of the EMT is the "pusher" end.
Put power on the motor, and it goes up/down. Had plenty of force too. All told maybe $20 or so worth of stuff.
OR, a boat winch crank with the cable running up and over the "inside" walls to lift up on the "outside" walls.
2 air springs are still the easiest thing to do.