There is a shower drain trap access panel somewhere under the shower basin. If your basin drain is towards the center of the camper, look for a small access panel with 4 screws on it inside the bathroom at the floor level near the drain. This cover is nothing more then a piece of 1/8" wall board with plastic edges around the board and 4 screws holding it on.
If the drain is on the left outside wall end, then go under the camper and look up at the area where the drain would be. There should be an access cover there to get to service the drain. This may be a black plastic covered piece of wood screwed on, or someone may have taped over the hole. The size of the hole under the camper may be able 6" x 8" more or less.
Once you take that access panel off, you may be able see the hot and cold water pipes. And the drain trap. It sounds like you have a leak in the hot or cold water line. It can be a fitting leak or a burst pipe.
Is this the first time you used it since de-winterizing it? Assuming you owned the camper last year and had it winterized. A pipe may have froze over the winter.
That is a place to start. If you get stuck, post pics of what you can see inside the access panel and where you see water on the floor and tell us a little about the camper and how long you owned it and when was the last time it worked OK. We do not know if you just bought it and it doesn't work or you had a long time and now it just started leaking.
Hope this helps
Current Sunlines: 2004 T310SR, 2004 T1950, 2004 T2475, 2007 T2499, 2004 T317SR
Prior Sunlines: 2004 T2499 - Fern Blue
2005 Ford F350 Lariat, 6.8L V10 W/ 4.10 rear axle, CC, Short Bed, SRW. Reese HP trunnion bar hitch W/ HP DC
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