So if the shower is on a common wall (back to back?) with the sink where the wet spot is found? If the shower handle is on that same common wall, check the shower faucet. May have to look for a plate in the medicine cabinet over the sink or on the wall above the sink at the shower faucet level. One or our trailers had the shower faucet fail and out access panel was in a pantry in the kitchen (shower wall was shared with the forward kitchen wall). Replaced the faucet assembly and all was right with the world again.