We had this problem with a prior boat. The underside of the mattress was next to the coldest part of the boat and, with hot bodies on top giving off moisture as we humans do, the underside was always moist. It even developed mildew. The simple fix was to put cardboard under the mattress (several boxes being thrown out by a local store, cut to shape). The cardboard provides sufficient insulation between the bottom of the mattress and the cold surface below, although I'm not sure what would be under your bed that would cause it to be cold.