parents want to use our caravan

my parents used to be caravanners and now in there early 70s dont tow anymore. We would like to let them use our caravan every now and again, and it would probably only be a few days a year.

obviously i would tow the van and prob have a few nights, then they would take over whilst we went home. I would then fetch the caravan when they wanted to come home. Question is, would they have to be full members and how would we book eg we have 2 nights then they have 2. we would not be all on site overnight.