you want a macerator pump, probably on a switch.
leaving water long term in an unheated bus is a recipe for disaster if you are not in a frost free area. you'll want a water system thats easy to drain/ blow out / winterize.
my kitchen and bath are on opposite sides of the bus, with a plumbed gravity connection. i think the cross over is in a spot just ahead of the differential and clears the drive shaft easy. i had to make a heat sheild for protection from my muffler too.
my waste tank hangs pretty low (its under my fuel tank), but you can see in the picture, a 2x4, that i used as a drop across the width of the bus. one side is at the frame rail, this side is the frame rail plus the 2x4