Just got the FC out of the garage to set it up for the first trip away at beginning of July. I was a little worried opening it up as we took a chance and left the mattresses in over the winter. Previously we,ve always taken all the soft furnishings out as I was worried about mice etc using as a winter home. Anyway everythings OK so we may risk leaving the settee cushions in next winter. It would make it easier putting it away at the end of the season. Think I,ll still continue to take things out like paper kitchen towel etc
I bought a small dehumidifier from ebay, left that inside and used it overnight every other night through winter, also bought a pond liner to cover with, if it will keep a pond in, it will keep rain and damp out.
Outside all winter and dry as a bone inside, even cushions.
Unless you can hermetically seal your camper, a dehumidifier will only take humidity from the circulating air i.e. from the environment ... so they can't really do anything for an 'open' space (open to outside air).
The best method is to ensure that air circulation is possible, and keep direct wet/damp off the camper ...
is why your system works TGC.