My water tank is currently fitted under the bed at the rear end of the Hiace camper.

I would like to fit one under the floor and remove the present one completly. The filling port is on the outside of the body and can be re routed to a new underfloor tank. The pump is a manual hand type. 

Anyone out there got any ideas on availability of tanks,  problems in fitting,  positioning,  electrical pump types, wiring etc. !!!!

I have the ability to build and fit stainless tanks. But I wonder if having ready built/off the shelf ones fitted is  better and more economical in the long run.

Also reserve under floor fuel tank for Diesel. 
I don't  realy want to carry a Jerry can of  diesel  on the rear end. As I  want to build and fit  utility box (swing out)  in that position.
Any ideas, advice, pictures, would be a great Help

Regards
Mike and Ellie

if you have the capability of constructing one yourself then you are halfway there is your spare tyre still in the original under rear position, if so removal of this opens up a vast space more than capable of taking a water tank of adequate size, the spare then goes to the front bullbar, a diesel tank underneath will take a lot more figuring out, I have noticed on ours that we have a long "space" or cavaty on the passenger side centre and have often thought an extra tank (water or fuel) would be able to be constructed to suit this area, there is aonly a couple of pipes that would have to be rerouted (exhaust and brakes) but I couldnt see any major obstacles sometimes the "off the shelf" ones are cheaper but they will not necesarily fit your needs, always better to build I have found, I build "rotomoulded plastics", steel and stainless if you construct a mould and just have a look on the net you could easily construct yours out of plastic if you wish, it's not hard, I prefer stainless simply for its durability