ive been thinking about getting a diesel four stroke generator, and having a home made calorifier setup, with 8 or 16litre stainless steel electric water boiler/tea urn.
With a plumbing fitting at the bottom to connect some 6mm copper tubing to the inside,with several meters spiralling inside the urn to the top, and out a hole in the lid.
Then with your pump to microbore connected to the copper tubing at the bottom and out with some microbore at the top with jubilee clip. So the water flows through the copper tubing whether the urn is on or not. So you can use it cold or hot.
If you want it hot, start up the generator which you could sound insulated in the back of the van with exhaust going outside, or have in a sounds insulated tube box with a electric fan behind the generator to cool it, then the urn will boil the water using 2200w (16ltr version) and go into keep hot mode at low power 150w.
The spiralling copper tubing will absorb the heat and heat the water as it flows through the pipe, having enough copper tubing in the urn will get your pure water quite hot, without having to travel through the pump and without coming in contact with the heating element, which is concealed and stopping most water contamination. You could have a Di after the water is heated to ensure purity. then theres no safety issues like with gas heating.
think it will work?