Almost all caravans are based on a sandwich system.
Outer skin is either Glass fibre, PVC (Type) or Aluminium, with a fibre filling, framed with soft wood, and a ply interior.
So to cut the answer down, in the trade, the wall boards are ready decorated with paper, if you want to cover this, you can either paper or paint with a water based emulsion.
It helps the moisture evaporate.
Cupboards/wardrobes etc, dont unless they are very bad, and any metal work, I would suggest a quick drying internal silk/eggshell or gloss.
But remember whatever you do in the confined space will linger for quite a while, so try to use safe, low odour alternatives, and a gental air type heat to aid setting.
Mike young, March 2009