We stored our trailer in Lake Havasu for 3 months at a Uhaul inside storage facility. Individual garages. It was $240/month. We wanted something out of the heat.
If I had it to do again I would try to find a private campground to park it. By the way, Lake Havasu is not the place to try to find storage.
Unhook your batteries (remember, black wires first!) so they don't have any parasitic drain. Take ALL food out of the trailer except can goods. My trailer was parked on concrete with mouse-proof compartments and sealing door. We removed all the food. My wife cleaned all the food storage compartments of any spillage or crumbs. You may be on the ground. Ants, mice, moose . . . .
$240 was on the low side for this area, but it gives you an idea of pricing. Work out a deal with a nice campground owner and maybe he will check on it day-to-day!