I am sure it is a Personal property tax. Just paid my NC tax on my 2014 TT. The system in NC now ties PP Tax with your registration renewal.
In order to renew my registration I had to pay the tax. Same now with a motor vehicle.
I have a boat trailer that I use only at my boat ramp and not on the road so I do not renew the registration, therefore I pay no PP tax on it. I think the
same would apply to a RV.
Not sure if they would make you catch up on PP tax you missed if you later renew the registration.
I have rental MH too and there is a PP tax on them, but it is paid through the tax office. No DMV renewal required on MH as they are not on the road. If your RV is permanent on the lot this may be how you are being taxed. Only way they would know about your RV is 1- you renew registration thru DMV 2- you listed on your PP listing as a permanent RV 3- your CG was kind enough to tell the tax office about it