I've been using the the limestone mine in Wampum since 2003. The first seven years was in the general storage area, since 2010 I opted for the climate control storage area. I used charcoal and DampRid while in the general area and never had a problem, I've used nothing in the climate controlled area and have no issues.
2016 - 2017 season was $155.00/mo. for my 40' DP. I've been very satisfied with the underground storage because it eliminates the need to winterize, protects the roof from the expansion/contraction that promotes leaking, and eliminates the damage of UV rays for the five months the rig is in the cave. $155.00 / month may not be bargain basemment pricing, but well worth protecting the large investment you've made in your unit.
