Well this isnt a permanent solution but its cheap and will give you a good idea if hot water is right for and if you want to spend more money on a proper solution:
What you will need.......sounds like Blue Peter:
1. a bath
2. submersible pump
3. hose to vehicle
4. and maybe insulation
So before you start work fill your bath up slowly ( the slower the hotter it gets ) then sub pump it to your water tank in your van. My central heating system is capable of heating a bath full ( probably 250ltrs ) to 60 degrees !!!!! There we go, an easy solution for some hot water but remember that the tds from the hot tap is sometimes a 20-50% more impure than your cold water because hot water will collect impurities from your pipes easier, and to try keeping that heat all day you will need to insulate your tank. Another thing to remember is the larger the body of water the easier it holds its temperature, so 2 bathfulls will take longer to cool down, Luke