Look for room under the bed of your pickup that is to one side or the other of the driveshaft, and away from suspension components, hoses, tubes and such. It is very possible that you can suspend a tank there using padded steel straps that are bolted to the bed floor, similar to an RV or fuel tank mounting. I have done this very successfully in the past.
The type of tank or container you use depends on what will fit. Use plastic, but it has to be rugged. An RV tank is a safe bet, and they are made in many sizes.
You will need a drain valve mounted down low, or a pump to bring it up to your camper. You will also need to rig up both fill and vent lines.
The beauty of this is that the weight is carried low, and in what would otherwise be wasted space.
Food for thought, if nothing else.