Check out a few facilities. If it is outside storage and someone isn't onsite 24/7, make sure they have cameras on your unit. Make sure there are other RVs and/boats on site. If there aren't, there might be a reason for that.
We have 3 storage options within 5 minutes of our house. They are not the same. The first one we stored at, our HTT got broken into after 6 months there. Haven't had a problem in the last 6 years at the place we are at now. This place has cameras, the gate is locked from 9 pm to 7 am at night and it is along a US Highway.
-Michael