A habitation area checklist should be something that we could easily develop with all the knowlegde available here. There must be one on the www but I'm too lazy to Google. Theres probably a need for an integrated campervan and coachbuilt checklist.
I'd start at the top and work downwards. Here's my effort to be added to........
Watertight roof, walls, floor, windows and vents. No rotten smells. Look for lots of silicone sealant at joints and the roof gutter.
All lights work on
battery and mains power.
Stove, fridge, water heater, space heater work on diesel/gas/12v/240V or a combination of.
Water pump(s) work on all taps.
Shower door does not leak.
Bench tops not peeling.
Sinks sealed into benchtops. Look under all benchtops looking for rot.
Electrolux/Dometic absorbtion type fridges must be fitted correctly to work - no space on either side, rear of fridge baffled with a top and bottom vent to direct heat out of the vehicle and away from the fridge.
Check flooring in all cupboards.
Gas bottles/tanks not older than 10 years or have a new test.
Beds rollout/fold easily.
No mould under mattresses.
Blinds work.
Windows and vents open and close and lock.
Underneath - waste tank taps work.
Toilet cassette (if fitted) can be removed.
Toilet dump valve works (if fitted).
Water tank fill point OK and clean - no smell of diesel.
Door locks.
Appliances that run on gas have exhaust vents away from any window that opens or has an interlock on the window to prevent gassing.
In general look for an interior fitout that has rounded cutouts for doors and draws from single pieces of wood/ply/plastic as these will be much stronger and lighter than square framed cupboards.