If you checked the windows when you last left your vav at the storage and no damage was present then it has to have been done while in storage. how long is it since you last visited your van from dropping it off, I know you said the owner has cctv but he is not going to sit watching it 24/7 and for him to say no nothing has happened well unless he has gone over every min of cctv footage he cant say nothing has happened. now im not sure what type of cctv he has it could be vcr recorded if so how often does he keep the recordings. we have cctv at work and it is stored on a hard drive to we can go back months and months of footage but without an idea of time and date its hard to find anything.
Is the storage a secure compound or just an outside storage with cctv on one part, if the 2nd well I think it maybe worth reporting to the police as attempted breakin giving the dates you last left the van to finding the damage, you will be given a police crime number so you can pass it onto your insurance company. There is some debate about wether the storage should pay for it as you are paying them to look after your property but well who really stands a chance of getting that and it would then increase you storage fees. But they should have an insurence policy covering damage to property and public liability, thats if you want to make enquiries down that road.