No, you are not being unreasonable. We have a cottage in Devon and it is on a water meter, costs a lot in council tax and the calor gas to heat it and to heat the water is eye watering expensive. If someone stayed there we'd be paying between £200 and £400 for their week's holiday depending on the season and how much heat they needed.
People are indeed very cheeky and have a curmugeonly attitude towards the possessions of others. We have heard horror stories of how properties lent to others have been treated and it almost always ends in such resentment between parties that the friendship is lost.
Having said don't lend your caravan, I would be very tactful about refusing to hand it over. Rather than a blunt, 'no', it would be better to hae some soft excuses. For example, it could always be 'being borrowed' for the dates your requesters want it. After a while a series of soft 'refusals' will get the message through. Another way to make it less than desirable is to ask for a series of favours done when the requester is there (cleaning, shopping , repairs etc.) Have a sheet printed out with a list of the 'useful numbers' the requesters will need while at the site but also add a tariff of prices for utilities/ cleaning (they can pay a cleaner or do it themselves)/ bills etc.