We had the same problem in 1997 when we went fulltime and we were fortunate to have had family, both parents and children who took our things "on loan." We still ended up with some electronics, ski and scuba gear, the one box of books we had left, and a few other bulky items that all fit in a 5X10 public storage building rental room. Not climate controlled. It got so hot in summer it destroyed the belts inside a tape player and caused damage from humidity and even the cold. At only 30 bucks a month, in three years we spent $1080.00. Since we wintered here on family property, we bought a much better alternative in 2000. A 12 X 20 metal on wood frame storage building, prebuilt on skids. We finished it out, plumbed it for a washer and dryer for the few months in winter we were there, and I put in 100 amp power, as well as full R-19 batting insulation and paneling. We also built two lofts in it, as it was a barn style metal building, and a cedar closet with two doors under one loft for off season winter clothes etc.
I put in a 110 5000 btu window A/C unit in one wall, which kept it as cool as we chose to set it, in the hot and humid Louisiana summers at 100 degrees, and 90% humidity. In winter, just one 1500 watt ceramic electric heater kept it warm.
Since family was out there, in the country, daily to tend the goats and cattle, they kept an eye on it for power outages etc.
Now we have just decided to come off the road and pursue another dream, and that portable building is now sitting on our 5 acres and I am almost to a return on investment over a 5X10 non climate controlled storage room. Instead I have much more than 12X20 because of the lofts, all climate controlled, and it will now serve as my office and studio. My total cost in purchasing the building, and putting in all the electric etc, including carpet and vinyl flooring, paneling the curved ceiling and all the rest was under $2700.00 At that time the building delivered was only $2000.00. Of course I did all the work myself, so no labor for the finish work.
If any of your kids or friends has enough space on their own property, for a portable building, where you can get at least power to it for the heat and A/C, you'll realize a return on investment in about one year over the cost of climate controlled storage.
I personally would not store antiques or family pics in non-climate controlled storage. If it is worth keeping for the future, and you fulltime at least six years like we did, the antiques would really suffer over that time period. Not to mention the potential damage from rodents and insects.
I know it is a bit up front, but it will serve you down the road as well. If it is on a family member or friend's property, you could sign a deal for however many years you think you'll need the building there, and in return for their space, you could give them the building. Makes sense when you compare six years climate controlled storage prices. You would have gotten ROI plus a profit, plus your stuff would be in the same shape as when you stored it, years later. Ours was so well insulated that the electric bill was less than 15 dollars a month, summer or winter. It cost nothing in spring and fall as the thermostats never kicked them in.
Not possible for everybody, but another alternative you may not have considered.
I have pics of the building inside and out I can send in email if you want. Just email me.