Ed,

One point which is not very intuitive is that GFI breakers do not need or
use the ground wire in order to operate; in fact they are commonly
installed in older houses which do not have the ground wire.  But the
question about the van ground does relate to the possible path of any
stray leakage current.

I understand you are not at the campground now & probably have no
convenient way to repeat the problem, but in my opinion, you did discover
a possible safety problem, and it should be tracked down or eliminated.
Is there any way you can rig up a GFI socket for 115V or run an extension
to one?

David Beierl indicated that Westys have breakers at the input.  I'd start
with that in the off position and then go from there.

I don't see why there should be any concern about problems between the 12V
and AC systems grounds, although I don't know what's wrong with your
system either.  We can speculate til the cows come home but without
further information, there's no point.

Regards,
Jack