There's really not enough info to help here, the reply above pretty much says as much as anyone can without more details.
But, there's a fairly good chance that one of your copper lines has corroded through. Have you looked at all the places you can see without disassembling stuff? Put the pressure back on it and look underneath the bathroom and galley sinks, along the line on the street side that comes from your fresh water tank. Drain the black and grey tanks to make sure you don't have a leak there.
Chances are though you are about to become familiar with PEX, ABS, or both. 41 years is a pretty good run for plumbing in your house, and your house isn't going down the road at 65mph. Once you start looking you'll probably find it's been leaking much longer than you think and there's rot in the floors.
Sorry to be the bearer of bad news, hopefully it will just be an open drain valve but be ready to dig into your trailer and replace all the plumbing if you want it reliable and trouble free....