I can give you some general advice about 70's trailer freshwater tank configurations. Schematics of your systems are available in the maintenance manual for your trailer's year. I bought a manual for my '73 off of ebay, there are a few threads discussing other sources for them.
Anyway, locate your fresh water fill door. From the fill door, there will be a large hose leading down to your fresh water tank on the inside of the trailer. This will be behind a gaucho or under some cabinetry. There will be a hole in your floor through which the fresh water fill hose passes, and the water pump is usually in this same area. Look down into the hole, and you will see part of your fresh water tank, and the place where all of the connections are made to the tank. This is the most likely place for you to have a leak.
Maybe all of you those connections are solid, the area around the connections is not wet, and the leak appears to be coming from the opposite side of the trailer. In this case, you might conclude that the fresh water tank itself is leaking. The only way to get a look at the whole tank is to drop it from underneath. First step is to drain the tank completely and to remove all connections to it. Next, you will get under the trailer and observe that just in front of the axles will be a portion of the belly pan that hangs down a few inches compared to the rest, and is framed in with angle iron. The front-most section of angle will unbolt from the rest, revealing a 1" thick piece of plywood that supports the tank and has to be slid forward out of the angle iron frame. This is usually a major operatioin, as the plywood is likely rotten, swelled, and rusted in place. Be ready with a floor jack to help support the tank as you pull the plywood out from underneath.
I would recommend thoroughly inspecting all of the hose connections before dropping the tank.