One idea I had ages ago and still haven't gotten round to trying is solar heating.
The first step would be to insulate your van tank and static tank very very well. The foil sided polyurethane foam rigid builders board is very good for this, and easy to use.
Next, an evacuated tube solar panel (These work well in cold and overcast weather, and only cost about 10% more than a normal panel), pump and controller. Set-up the solar panel on the roof of your shed, and let it heat your static tank as much as possible. You'll probably get 1000 litres up to 70-80 degrees C if you've insulated right.
In the morning, transfer the hot water to your van tank,.. and when you get home at night transfer any remaining back to re-heat.
The advantages,... no extra equipment in your van, no gas cylinder (which would have to be declared to your insurance provider),.. no extra fuel costs, so it would pay for itself in 6 months to a year,.... and you'll get a lot of extra customers by pushing the green eco image in your advertising.
The disadvantages,.... more expensive initially,... more work involved (A good bit of DIY probably),... a gas heater would get the water a bit hotter,.. your tank will cool slightly over the day.