To get back to the OP's original question after all the crying about how insecure campers are; there are often long spaces under a bed or couch or in a closet where you could install a long locking metal box. True, it won't give you gun safe level security, but if bolted or chained to the chassis, it would take more time to open or remove than most burglars would be willing to spend. If you're parking in an RV park, as opposed to leaving it in a storage lot, there will be neighbors around to further inhibit explorations by the criminally curious.