[JRTJH]

If you locate the water pump, you'll see a 1/2" line "into the pump" (from the fresh water tank) and a 1/2" line "out of the pump" that goes through a wall, into a cupboard and then to "destinations in the trailer"....

Typically, water lines are "above the floor" (to protect them from freezing) so you should be able to open cupboards, pull out drawers and remove cushions/plywood platforms to access almost all of the fresh water lines and fittings.

As for finding a "plumbing diagram" or "schematic".... you might as well not waste your time looking. There simply isn't one and Keystone has chosen to not provide that information to customers, so you won't be able to "appeal with crying eyes" to Keystone to get any information.

It's pretty much a situation where you need to hire someone to find the leak or take the initiative to find it yourself. Either way, don't look to Keystone for any "technical help".... There are dozens of accounts on this forum posted by members who have tried to get help in the form of technical drawings, all of them have failed.