I have some experience with some of the suggestions that have been made 
for adding hot water for a shower to a Vanagon. My general advise is BE 
CAREFUL!

In an old '73 Dodge van I made a hot water tank from a short piece of 3" 
copper pipe. Through the end cap I silver soldered a coil of 3/8" copper
tubbing which connected with tees to the cooling system. Then I carefully
insulated the tank so the water would stay hot. It was terrible! The water
got much tooooooo hot. (Makes sense, the coolant can be 200 degrees plus.)
Turn on the faucet, and get a blast of scalding hot water. (A great
pressure build up would occur in the tank.) I tried mixing the output with
cold from the tank, but the hot was under so much more pressure, that that
never worked either. It was much too dangerous without a thermostat to
shut off the coolant loop to the water heater when the proper (125 degree
max) temperature is reached. I never found the right thermostat. 

There is plenty of heat there, but it is difficult to contro.

David Garth 
86 Westy
San Luis Obispo, CA