The Camping and Caravan Club. I have two storage sites listed on my policy,one here in the U.K and one in France. We may go across to France in March or April, Do some touring, then leave the Van at a storage site in France, go home with out the van and then return 6-8 weeks later, do some more touring, back home then bring it home in the Autumn. Make sure you are completly truthful about the storage site, e.g clearly defined perimeter, Staff on site etc,etc. The caravan club may do the same.
Von Ryan