I have never actually camped there myself, but I gather parts of it are rather open to the general public as a walkthrough. Whilst I would suspect that if it is busy, there should be no real problems, the fact that you can lock a motorhome would make it far more secure than a tent. Added to this the fact that if your motorhome has toilet facilities, then you should be laughing.
There is a bit more info about the various campsites on Beermountains excellent site
Click Here for Camping GuideI should also add, that whilst camping my group have had our tents turned over in Maison Blanche, and the group next to us were once done in Houx as well, so it is something liable to happen in any site. Always keep your valuables locked in a vehicle, and some spare clothes. The little toerags will nick almost anything including food, cooking utensils, clothes and of course alcohol