Have just gone through this for a 40' DP in the Surprise area. There are lots of storage places, but they are all rented. It took two weeks of calling 15 to 16 places every day until we found one. None of them would used a waiting list, they said we had to call every day.
We even briefly looked at enclosed space but at well over $600 a month we quickly ruled that out. Because spots are so hard to find, what current renters are doing is to continue to rent them for the winter months while they have their MH out using it. This assures them of a spot when they're ready to put their MH back in storage.
When we found a spot we rented it within a hour even though we wouldn't need it for three weeks.
Good luck.
Bill
Nodwell RN110 out moose hunting. 4-53 Detroit, Clark 5 spd, 40" wide tracks, 10:00x20 tires, 16,000# capacity, 22,000# weight. You know the mud is getting deep when it's coming in the doors.