Try everything electrical you can, ask about heating and water systems, and decide if its been lucky enough to enjoy winter storage out of the elements or has been damaged in the past

These excellent campers don't suffer a great number of issues,
Legs; as powered can be problematic if rain has corroded mechanism, requiring a rebuild,
Leaks; usually to rear wall, (toilet) sometimes in cab-over (at join with headboard,) once with a members' roof (though uncommon)