There used be rigs available to carry a boat above a truck bed.I believe even with a camper shell in place.There was a winch to launch & retrieve the boat as I recall.Search the internet to see if still available if interested.
Old-Biscuit has the right idea in terms of cost and usefulness"Buy a Flojet Portable Macerator Pump and a 3/4" garden hose in length needed to reach the cleanoutThen Dump/pimp your Waste tanks...black then grey into the clean out"
