You came to the right place because I am about 2 years ahead of you figuring out that everything breaks at the 10-15 year mark on a boat. You have a leak and it could be from 20+ different places. You just have to go through it and eliminate them one by one until you find it. And in my case there were three leaks, so the first and not even the second discovery solved it. My leaks were all at the fittings. The main branch fittings from the 22mm supply lines are in two places. There are a bunch next to the shower sump in the hall floor compartment. I had no leaks there. And there are a few under the starboard bunk in the mid-stateroom to feed the water to the galley sink. Lift up that hinged bunk and remove the aft floor panel. You will find fittings there and the cabin sump pump just aft of that. If that sump has water in it then you most likely have a leak forward of that and its running back. Anyway, both my fittings there were leaking and I had to replace them. The original fittings are made by Guest. I replaced them with SeaTech fittings from www.freshwatersystems.com. I also had a leak at one of the fittings in the trunk behind the panel where the shore water inlet is. To access that I had to cut a 4" hole in the panel (which I then put a cover plate over). There are also obviously fittings on both sides of the pumps and at all the sinks and toilets, showers and anchor washdown.
