Looking at your floor plan which is similar to mine. There are several possibilities. Your outside city water inlet is behind refrig. your outside shower, if equipped is in same area. All pluming running to kitchen sink runs through that area also. Possibility, if not winterized properly, you could have ea line leak in that area. I would pressurize system and do a leak down test. If it leaks down you have a crack somewhere in your water system. if leak down test shows ok then you have water intrusion from outside. A leaking roof seam, a hole in roof membrane or a leaking window, or vent.
Jim and DW
2001 T-2553 Sunline Solaris
2006 GMC Sierra Duramax 2500HD 4X4
Firestone Transforce AT tires
Reese Dual Cam Sway Control