Kevin, set this up myself a few years ago, I bought a
Torbeck water valve (side fitting) to fit into the side of my then watercarrier - I've got an Aquaroll with the valve set into the sidee opening cap now. A hosepipe with connector between inlet on Torbeck valve and water tap.
The simple process is that when cold water reaches the height of the valve the water stops - as in a toilet cistern. The submersible pump then takes water into the caravan in the normal manner and the simple system keeps the water topped.
For the waste water I bought some rigid domestic overfolw pipe 1" dia and a collection of straight and elbow fittings. I made a simple modification to the waste outlet which allows me to push the 1" pipe directly into the waste outlet of the caravan,
By joining lengths together I can position the end over the drain. I have a junior hacksaw to cut pipes as required for length. Finally I ensure a gentle slope on the wastepipe by packing it up with bits of stone, wood or whatever, to give water flow. This lot takes a few minutes to set up but as I never have to carry any clean or waste water it's well worth the effort. Total cost