Earlier this year I checked out a cave near here that offered RV storage. The temperature is about 57 year around. Humidity is lower in the winter, running about 70-75%, than the summer when it was above 80%. They had huge sheets of plastic in places to help manage the humidity. I took a tour and saw some interesting sites. I saw an old Volkswagen Thing, sitting for more than 15 years. It was all dusty but looked OK. There were all sorts of trucks, RVs and cars. The manager said they had a lot of long term storage because of the constant temperature and secure parking.
The drawbacks, at least in the case of this particular cave, were you aren't allowed in there by yourself, you had to be escorted to your vehicle and you weren't allowed to work on it because the escort had to stay with you for security purposes, so you couldn't get into others' equipment. There was no power and no water. You had to notify them about a day or two in advance so they could arrange for the escort. You had to pay each time you took it out. The manager said it was really for winter storage with people finding another location for summers.
It was a pretty steep entrance angle from the street down to the cave entrance. It started snowing and the manager said we needed to leave because if it snowed hard enough we wouldn't be able to get up the hill because of the mud and snow.
We opted for commercial storage with a resident watchman. Another AS just moved in so now ours doesn't feel so lonely.
The cave was dry, dusty, but dry.