Mike,
There is a line in the FAA student pilots manual that I have always liked, it says the only time to believe a fuel gauge is when it shows empty.
There is a issue with many RVs. The tank placement makes fueling difficult. And now, with the high foaming gasahol (aka motorfuel) things are we worse than we ever expected. A fuel delivery nozzle detects full by using the passing stream to suck in air. When anything interferes with that air flow, it trips. So, the foam will get you. This makes fueling a flat tank to full very problematic. And worse yet, some pumps are more sensitive than others.
Then there is another problem. The hardware produced in those days was not as alcohol proof as they had hoped. There is no reason to trust the gauge reading as the calibration could easily be bad.
My solution? Know your coach.
This is easier than it sounds. Write the odometer reading on every fuel slip. (Yes, you have to push the "I want a receipt button.") Use the spreadsheet in you laptop to keep a running fuel average. When the last fill would say you were getting 20 MPG, you know it was a bad fill. Just stop a little sooner next time.
Matt