I once owned a 1972 Ford van that had been customized by a college student who drove it to do his graduate work in botany in Guatamala. Except for the floor and windows (only had front & back & two door windows), it was totally insulated and paneled.

It had a crank-up roof vent that was okay in rain as long as you weren't driving. I drove all over the western U.S. in that thing, and was as comfy as could be.

For a trailer, I would have fold-down platform bunks on hinges & steel cable (or chain). This would give you more storage room, but could be easily converted to living quarters.

Ventilation is important. Heat rises. I've had a van with a roof vent and one without (for a short time), and the difference is incredible. Vents must be screened.

Screened windows that can be secured from inside with solid covers. Gun barrel slot optional.

Anchor as much as you can so it stays secure. Having to rearrange everything so you can sleep after a hard, tense day is a real PITA. (I learned this after the near-deer event in ID.) It doesn't have to be permanent, just anchored. Like the 12x18x5" FAK that clamps to the wall right inside the door, but can be released so you can take it with you.

Don't waste the storage space near the roof, along the walls.

A roof rack can come in handy. Put some thought into it so there is support all across the roof itself, not like those stupid station wagon rack things.

Put a relatively flat water tank on top of the roof, painted black. Sunny day = warm water without wasting fuel.

Dual purpose is always good. A fold-down plywood bed platform can also serve as a dining table. Square cushions on storage boxes make more comfortable seats.

Check out boat equipment for fasteners. What they don't have for keeping things secure while moving probably hasn't been invented yet. It's not all expensive.

Consider some of those boat things that look like a metal dimple with a ring that swings up so you can tie things to the outside. (I've duct-taped 10-ft lengths of plastic pipe to the side of a compact car, but there ARE better ways.)

More of those dimple/ring things all over the ceiling could come in handy.

Seal all seams with EXTREME care.

Sue