I think you are making the right move. Sounds almost if the current site was blackmailing you. If there ever was an unfair clause in a consumer contract, that was it regarding servicing!
However they have taken the choice away from you and what do you do if the service is poor quality or your caravan needs warranty work? If you complain, you may be asked to leave.
The site didn't take choice away, The caravanner had the choice to use the site with service or take their caravan elsewhere. For whatever reason the OP chose to use this site. It was a free choice to agree to this contract.
What do you do if a service is not up to the standard you require? You follow the companies complaint procedure and if that fails you use the CRA.
As was explained earlier in the thread Service work and Warranty work are entirely differnt, and governed by their own T&C's. No mention has been made of any restrictions regarding having Warranty work carried out.
The OP has clearly explained several times that they were happy to use this site and its servicing element. Suited their needs and wants at the time. Their needs have now changed and they have decided to move, the site has not terminated the contract.
As far as I can see there has been nothing underhand or unfair playing out. The site owners have seen a way to bolster their business by offering the service element, and clearly there are many caravanners who also find the offer to be attractive enough.
It will obviously vary from person to person, but the convenience of not having to take time of work and the cost of towing a caravan could easily add up to more that difference in servicing costs.
To use Beachballs signature comment - To Each His Own....