I was checking into this very setup a while back and I was thinking of an economical way to pump the water that didn't take elecricity. I don't think there's enough "head" to just gravity feed from a water container on the roof rack and get any satisfaction from the shower.
I did come up with two as-yet untested methods, though:
1) Using a Scepter water can, fashion a schrader valve onto the vent port so that you can pressurize the water can using your onboard air tank, forcing the water out through the drain port or
2) Using a Scepter water can, fashion a barbed fitting onto the vent port so that you can attach one of those insect sprayer tanks (the ones that you pressurize with the built-in hand pump). Then pump it up, pressurizing the system as desired, forcing the water out through the drain port.
Of course, in either system, you should have a pickup tube going from the drain port to the bottom of the water tank in order to use the system upright.
I was thinking up these shower-related things to make extended minimalist vehicle-type camping/exploring seem a little more appealing to my wife, though I suspect it's a lost cause. Give me a moistened face cloth and a full stick of deodorant and I'm good for weeks.
BTW gasman---5 women?? I think I know what your expedition trailer looks like: