Have you ever replaced the fuel filter?  You leaving a gas station sends up
red flags to me about bad gas or water in the gas.  So if you have never
replaced the fuel filter, I would do it and then see what happens.
As for the gas mileage, have you ever replaced your oxygen sensor?  They are
only good for 50k miles (I think).  A bad oxygen sensor can really trash your
gas mileage.
Remember grasshopper, if you are to own a Vanagon you must learn to
troubleshoot.
Those Bentleys are usually good guides for that (I have only looked at the
ones for Jettas, never seen the Vanagon one yet <my dream>).
Have fun.
Ken Wilford
John 3:16