A cheap 12v electric pump and some hose. I used a medium-sized marine bilge pump when we used to do that often. Just make sure the pump is rated to lift to the height you need. 4 ft is probably enough, and many bilge or wash pumps can do that and they aren't expensive. If you want to move water higher, or faster, get an RV water pump, but that's more bucks.
Former owner of 2004 American Tradition and several other RVs
Home is West Palm Beach, FL