A friend of mine who does not use his camper too often and has it standing at his house while he is starting it every other day to charge the battery, went to the gas station with half a tank to fill up and could not get it going faster than 30 mph.  
I could not tell him what it may be, but I promised to ask the Vanagon professors.
Does any of you have an idea?  
To me it sounds like the connection at the AFM is off.  Or the catalitic converter is plugged by its own stuff.  
Who knows?
Zoltan