Is it the trailer the theives want, the load or both trailer and load?
So many people look at securing the trailer but protect the contents only with a sheet of canvas. The contents of many camping trailers is a lot more valuable than the trailer.
Just remeber that taking a trailer will be alot more expensive in crossings and fuel, hence we don't take ours just use a back box. When we did take it we have a lockable hitch which is invaluable and a hard top. Also, as stated many F1 hotels etc have gated car parks.
Amazing family weekend with old steam engines, classic car displays, market stalls, and full catering and bar. And camping on site - Save £25 by booking in advance.