I can offer a possible reason a boat or camper sits idle. The act of selling it signifies an end to any possibility of enjoying it again. As long as it is held onto, there remains hope for a return to camping/boating or whatever.
My mother was in a nursing home for years but refused to sell her house. As long as she had the house she could pretend to herself that someday she would go home and take up her life again. Others in her neighbourhood wondered why I kept going out there to maintain the property as I held power of attorney and could easily have sold it off, but that would have broken her heart.
Some of those trailers could be similar stories.