Before I got addicted to Vanagons (A westy, 2 gls and now a Syncro) my
brother had one that he had installed some kind of shower in.  I suggested
that he heat the water by running a type K flexible copper tubing around the
around the exhaust manifold and muffler, have a high and low inlet on the
water tank and let the water heat by convection.  Free heat from the muffler
no drain on the electrical system.  I guess you could add a small electric
pump with a back flow preventer to increase circulation.  He did it and said
it works great.  I don't have the particulars but could get them from him
since a lot of people seem interested in this subject.