Seems to me that you could test battery voltage, main + aux. battery, start
engine, re-test with engine running,
both should be higher. Shut off engine, disconnect one
terminal of main battery, turn on key, see if radio plays.
No radio = isolator is keeping aux battery out of circut
Radio plays = possible short in isolator
Re connect main battery, disconnect aux. battery, try a light in camper
No light = isolator OK
Light lights = short in isolator
This is best a guess as I've got to offer, knowing Chevys, I'd take a
close look at grounds, they are
skimpy at best, problematic at worst. A heavily painted alternator bracket
can cause all sorts of Chevy ground hell !