What alerted the city to the fact you have an RV registered at an address outside the city? Do you perhaps bring the rig into the city often and there were complaints from neighbors or it drew the attention of the local police? Regardless of whether or not it is in the city, you will still owe county taxes since all cities are in counties.
Like another poster, I cannot imagine a situation where the city portion of property tax is anywhere near $2200 on an RV. I looked up Aiken's mill rate and it is 62 mills. They assess real property at 4 percent of value, so the city taxes on $100,000 assessed valuation is $248.00. Is your RV really worth $900,000, or are they looking for multiple years of taxes plus penalties and interest, or is the $2200 tab a combined bill for city and county? County taxes are much higher than city, since they include the school taxes, which is the bulk of most state's property tax billing.
If the location outside the city limits allows for RVs to be occupied as residences, I would say you have a great case to beat the city. If the location is not currently approved for any habitable dwelling, your case is much weaker, since it would appear to be that you own that personal property and are attempting to circumvent the local taxes. It may even end up costing you more to store in the county than the city because you will surely owe full county taxes on the RV and there may be county resident mills that exceed those of the city. I know where I live in Montana, the residents of the incorporated towns actually pay less tax than the rural county residents because the rural county mills for things like waste disposal, Fire, Ambulance and public safety exceed the equivalent mills in the towns. It kind of makes sense, since rural density is much less than city, costs to provide those services can be higher. I don't think you are going to be able to shed both the city taxes and the county taxes.