I look at the electrical system. I have yet to find a bad RV with nicely done electrical (AC and DC) outfitting. Wire nuts, tape, snarls, over-stripped cables, and garbage grade fixtures are one good clue. Metering that belongs on a six-year old's electric mini car is yet another minus.
I wait until afternoon on a 90+F day then enter a closed up RV. If it's noticeably (surprisingly) cooler than outside, that too is a good sign.
Generators that sound like they're next door in a Motel 6 and vibrate like a coin operated bed are also bad Juju.
JMHO