Ok, something that is well known but widely ignored is that the rolling weight of your vehicle is a big contributor to it's speed and it's economy.  I read of people on the list who just seem to 'pile on' the extra stuff into/onto their vanagons.  Every 10 lbs makes a difference, and it is easy to forget that it all adds up quickly, bit by bit.  As I build my own current ride,  I've been trying to keep in mind how many new lbs. each addition will make.  I've already lost few mpg. with the addition of a partial Westy interior, a full water tank, extra battery and the propane. 
 It's real easy to forget, though, and it's tempting to think.."Hey, it'd be nice to have a _____ along.  Maybe I should find one on the Samba and stick that in here, too"  It's easy to let things 'build-up" inside, too.  A few days back I was under my fold down seat/bed and found some extra tire chains (about 20#} and some carpentry tools (another 50#)  I had my bikes in there and 50# of dog food, plus a full water tank and other 'stuff'.  Every start, stop, change of direction, I was 'moving' that un-needed weight around, wasting energy and using up fuel and time..
  After racing a Porsche for a few years ( ran out of $) I learned that even 10lbs extra has a quantifiable effect on how well the vehicle works..Just something to keep in mind as you pack up for spring vacation or do your 'renovation' to your vans...Surprisingly, adding horsepower is much less effective at helping performance than subtracting weight.  So maybe next time you get passed on the interstate, don't mutter about the underpowered engine, think about what kinda load you may have aboard..
  Don Hanson