Leak from shower in 2109s

[EmersonTT]

We discovered that our 2109s shower has been leaking -- a slow leak-- only when the cold water is turned on. I discovered a drip at the very back of the trailer, made a slit in the black fabric under the trailer and found that a section of the wood (plywood??) panel under the trailer to which the fabric appears to have been glued is saturated and coming apart. I checked under the shower (tough-- only room for your arm), and I think the leak is coming from the coupling close to the floor at the very back where the cold water feed is linked to the hose to the cold shower faucet and the hose to the toilet. Any ideas on how I can get back there to fix this? Should I try to replace the bad part of the wood or let it dry out and ignore it? Any thoughts are welcome.