Dougie, I see you are a new member, so probably haven't seen many of the "humidity level" threads that pop up quite often on this forum. Well, it doesn't really matter, I think this idea is a REALLY bad one. You don't say in which state you live, but that probably doesn't matter EITHER, the idea is still a dangerous one.
Humidity levels should stay roughly within 15 points either way of 40. You'll be swinging much more than that, plus if rain happens to leak into the storage unit and get baked by the midday sun, it'll be like a sauna in there. Temperature swings in homes is typically , what? 30 degrees or so MAX ? Vs possible 30 degree FAST drops with storms, and perhaps 90 or 100 degree longer term differences.
I'm not a tech and don't have any actual experiences with people doing this, but I bet there are some people here who do have horror stories about it.
You'd be better off storing it in a furniture warehouse, though the cost would be more and you'd still have to check about temp and humidity swings.
Perhaps you have a friend or a friend with a friend that would like to have a piano in their home for a year. Bad playing would be better than bad storing.
A church might be another possible place that could use it and would store it for you......second choice due to possible bad pounding, etc.
Check with your music store. They MIGHT store it for you for a reasonable fee.......certainly less than the $3500 I think you mentioned you paid for it.
There are people out there looking to rent pianos. You could match or undercut the local music store's rate and maybe MAKE some money on it. (not counting transportation off course). You might feel uneasy about leaving your piano with strangers, but you'd probably be better off with a couple of scratches than the freeze thaw torture test you'll have at the Storage Unit.
Then there's this joke:
A man walks into a bank and says he wants to borrow $200 for six
months. The loan officer asks him what kind of collateral he has.
The man says 'I've got a Rolls Royce -- keep it until the loan is
paid off -- here are the keys.'
Six months later the man comes into the bank, pays back the $200
loan, plus $10 interest, and regains possession of the Rolls
The loan officer asks him, 'Sir, if I may ask, why would a man
who drives a Rolls Royce need to borrow two hundred dollars?'
The man answers, 'I had to go to Europe for six months, and where
else could I store a Rolls Royce for that long for ten dollars?'
Oh well, maybe if you had a Faziolli.