Fresh waterline leaks

[Argosian]

I just bought a 1976 Argosy 28 motorhome; center twin with rear bath. Unfortunately the copper plumbing has numreous leaks. I've repaired the visible leaks but water still runs out the bottom from the belly pan, but it appears the water lines to the shower run around the back, below the bathroom sink and stay above the interior floor yet no water leaks into the interior. I don't have a compressor to pump air into the system to "listen" for where the leaks are. Do I need to drop the belly pan? Are there any water lines under the floor in the area of the gray and black water tanks? I have a reproduction of a 1978 owners manual, but no diagram for the plumbing system. Any help would be greatly appreciated. I'm competent enough to do the work on the plumbing but I don't know "what to tear up" to get at it. Thanks.