I sometimes think that this reflects the uselessness of fora. Someone ask how to do something and loads of comment comes in with why they shouldn't. The man wants to add a water tank, this is maybe a lifelong ambition or just an exercise in engineering and carrying 10 litres of water in the van (10kg) won't break anything. Personally I've just bought a used Bailey series 5 with fitted tank and think its the most useless addition one could get in a van. It takes up useful locker space. Not supposed to travel with it even partially full, doesn't give continuity of water supply without changeover valve operation etc. Nevertheless if you want to do this then copy the Bailey system with two 2 way and off valves and on board pump. You could use solenoid valves, and even Arduino control to effect automatic changeover from aquaroll to internal tank.