I would start looking at all of the plumbing valves. I do not know if your trailer has a single low point drain for the tank and the cold piping. It could have a valve that joins the systems together. Remember this is a caravaner, they did some different things on it. A fill valve to add fresh water to the tank while hooked up could be one of them.
I would start by diagramming all of the water lines, where they go, how they are connected, and how the water should flow. Once you do that I bet the answer will jump right out at you!
WBCCI #5501 AIR # 49
1978 Argosy 28 foot Motorhome
Wise men talk because they have something to say; fools, because they have to say something. -- Plato