How to keep gray water from coming up into tub?

The small motorhome in my signature has small holding tanks so we ran into the same issues that the op has. We bought a cargo rack that fits into the trailer hitch receiver. Then bought a 21 gallon portable waste tank and carry it on the cargo rack. Even when it is fairly full, we are still able to lift up one end onto the rack and then slide it all the way up and into the rack. We can extend our stay for a few days longer using the portable tank.