You might find some ideas over here:
I want to use a 50 gallon plastic barrel to run a toilet and sink. Is this possible? What kind of pipe and pump should I use and How do I hook it all up? Does anybody have a diagram.
I use a rainbarrel at our camp. It is a plastic drum, about a 55 gallon, that has conduit running into it from the spring house up on a hill, then, it runs down conduit into the sink in our kitchen. Works real well. Just once in a while got to go clean it out or fix the spring house. Nice soft water.
An RV "demand pump" which operates when the valve opens would be the easiest route.