Richard,

I happen to have a very unfortunate familiarity with this part of the coach.
If both fuel gauges went full at the same time, then there is an extremely high probability that if it is not the instrument, then the problem is between there and the selector switch. Diagnosis should be real easy. Just clip a lead on a good ground a probe the back of the instrument and then the common on the switch. One of those should make it say empty. If the problem is not there, I bet we can nail it down in short order, but it will require getting dirty.

Matt