Insurance for leaving a caravan in France

Hello, we are returners to caravanning after a 25 year break and have just bought a new caravan . We planned to take it to France touring in the Spring, and leave it there when we come back for a month or so (to avoid the school holidays). Then to return in early autumn and  bring it home. Only 2 Insurers out of lots including Red Pennant will quote for a van left in France. One wants £600 plus and the other £300. The van is valued at £14k. Whats the problem? Are lots of vans stolen in France? It will have an Alco wheel lock etc and be on an English run storage site.  Its beginning to worry us! Does anyone else out there do this. It will save loads on ferries/diesel and time.Any comments will be very helpful!  Thanks