DIY Water Tank for my van:
Not a Nissan, but might give you some ideas:
Valterra 12 Gallon Tank, place your fittings wherever you need them (drill and chemical weld):
Drain valve and hand-pump over the plastic-lined step at the slider door:
I've made provisions for an electric pump, but honestly I don't think I'll need it until/unless I get a hot-water setup (Helton) and want to take a shower. The hand-pump works great and helps the family conserve water.
More details of the tank process
in my build thread.
My air system is still portable...