Hard to guess how any particular company might set their rates. Motorhomes can usually get lower rates when not being driven, but that is a matter of removing the vehicle liability insurance, which is a very substantial piece of the total. For a towable RV, liability insurance is not a factor.
Statistically, there is probably a greater risk in having an RV sited in a campground 24/7 than having it at home between uses. Not sure how a storage lot changes that - some are much safer than others. That would be something to question the insurance agent very closely about, i.e. what, if any, difference in coverage and premium will there be if kept ay home vs a storage yard vs a campground.
And as Stu noted, in some areas a non-movable RV may be considered to be at greater risk. An RV parked year-around in Florida is basically just a residence that doesn't meet the building codes for the region and thus is at greater risk. Of course it could also get blown away if stored at a Florida home, but so many rigs in Florida have absentee owners that the statistics have to accommodate Rvs that are essentially abandoned to their fate. That may not be a factor in, say, NY or Wisconsin.