A water-filled horse trough outside the trailer with a bucket next to it is sounding more and more appealing.
When I was looking at how to move water for a gray water system the guy at work I spoke with compared the various systems he uses (at work he pumps printing chemicals, at home he has an above ground swimming pool and a large salt water fish tank). He said the easiest way I could solve my problem of tub water to a holding tank outside was to go to the pet or fish store and get a transfer pump like he uses when he wants to move some water out of his salt water tank to put new in. It wouldn't run all the time or when the pressure changed, you would turn it on, move water (like into that gravity feed system I suggested on outside the trailer high on a platform), and turn it off again.
Heck, Bobert, just get CREATIVE! Turn the toilet tank into a house water feature and put in one of these with a line from the trailer up the hill.