Just another point of view on the water tank:
Built-in water tanks have several problems - they are very hard to clean, they can taint the water, they can be difficult to refill. And the weight can't be shifted around.
Most importantly for expedition purposes, a single big water tank doesn't allow you to grade your water. Let's say you have half a tank of good drinking-grade water. What do you do when you reach a supply of rather dodgy looking water?
Opaque plastic water 20-litre containers of standard Nato dimensions are great. For pumped water systems, simply make or buy a modified cap, and use it with each can, one by one. Cleaning, refilling, draining, replacing, are all much easier, plus you can grade your water.
Ok, they don't fit down under the chassis somewhere...
Still, having tried both methods of storing water, I'd definitely vote for individual, portable containers.