I fill the tank and stop filling when the auto-shutoff occurs on the pump. When the tank is "full", I can usually drive about 40 miles until the needle on the gas gauge drops below the "full" line which equates to about 6 gallons (at 7 miles / gallon). This is how I first noticed the problem - I would take off on a trip and my needle would be below "full". On several occasions I immediately stopped and filled up the tank to determine how much gas was missing. It is pretty obvious when I store the motorhome for 6 weeks and the tank is only at 3/4 full after parking it w/ a full tank. I suspect is is evaporation, but can't figure out where/how it is evaporating from and I would also expect an engine fault light if I had some kind of hole in the top of my tank (which I am unable to see)...I had several people tell me that gas could/would not evaporate that fast. (as an experiment, I placed a gallon of gas into a bucket under my motorhome the other day and when I came back 10 hours later, about 75% of the gas in that bucket had evaporated....)
