I store my Motorhome in a Cassoa Gold Storage site with 24hr access via a digital card, which does reduce my insurance, but certainly not by anywhere near the amount I have to pay to store it. It's there more because it's way too big to leave outside my house without causing an issue with my neighbours.
I've never had or heard of any problems within my storage site.
A question for those who do use storage. Do you leave it outside? Do you use a cover and does the cover effect the bodywork/paintwork? I am debating the storage idea but would it be beneficial to pay for indoor storage/barn etc or go with a cover. In the past my vans have gone green fairly quickly when left.
Mine is outside, and under a cover. I've gone into detail on covers before, as it's part of what we deal with within my business, as in the damage they can cause, and rectifying this damage.
A quick easy rule of thumb for any cover use: Never cover a vehicle that's not perfectly clean. Always make sure the cover fits nice, and any loose parts of the cover are strapped up.
Follow these rules, and you won't have any issues using a cover, ignore either of these rules and you will need a very costly detailer to fix your issues.
As for indoor storage. The only issues i've seen or heard of are potential mice damage or bird poo if it's a barn/farm building. This is why I choose to store outside, and use a cover, where both of the above issues can also occur, along with many others, but it's a necessary part of my Motorhome ownership, and I long for the day we can find the right house that has the room to store my MH, as well as everything else.
Bare in mind that the issues are only a small amount of cases in the overall scheme of indoor storage, but it happens.