Walt, the original steel tank in our '63 leaked like a sieve, so it got tossed out the door, and replaced with a 15 gallon plastic tank. I plumbed the tank in so it could be either filled through the regular fill door with a hose, or you could lift the mattress and base for the bed, and turn a valve to fill the tank from inside while connected to city water. I installed a par water pump between the rest of the system and the water tank so it wouldn't overflow while hooked to city water. If you did that, your plumbing would be a lot simpler, as the pump has a check valve to keep that from happening.
Meddle not in the affairs of dragons, for you are crunchy, and taste good with ketchup.