I have been thinking about using a pump, also. Then my gray water tank could sit under one of the twin beds in our 57 Airstream Overlander. Both sinks are high enough that gravity would take care of them. I think that a simple 12v
pump (that can run dry), with a switch near the shower would work fine. The small pump could be situated just below the drain. As another poster said, one would want an alternative method for letting the water go out. It is a problem..for sure. Not enough room inside the framework under the shower. Let us know how it works if you do it.