What kind of RV? If towable, some states issue permanent plates, so there is only a one-time fee, but the plates for the tow vehicle might be relatively high or low. This state does not require any plate at all on trailers, including RVs, not used commercially or in agriculture, but try to explain that when you get stopped in another state.
For motorized RVs, states might tax on value or weight, or might just have a registration fee each year after collecting a sales or excise tax on issuing a title. My motorhome is registered as a passenger vehicle, tag fee is currently $45 (+ $1.50 to tag office for insurance verification). That's because of how long it has been registered, the tag is on a downward sliding scale that started at $120 the first three years. Excise tax was 3.25% of purchase price, paid once 12 years ago.
I would not focus on vehicle registration costs or other taxes in looking for a less expensive place to live, because the big driver for cost of living is land values, which in turn influence wages and taxes, thus cost of services. If you want to live on a budget, you live where land values are low.
There are rural areas, and some almost ghost towns, here and in East Texas, and across much of the Deep South, where you can rent a house for as little as $400 a month or buy one well under $50,000, so it might be cheaper than parking a RV. I suspect there might be similar bargains in much of the Southwest and rural Great Plains where population densities are low and the climate is too miserable to draw hordes of retirees.
How cheap rural living can be depends on what urban amenities you can do without. I know people who still live without electricity, telephone service, or running water, and the nearest grocery store or gas station is more than 30 miles away. Their living costs are quite low, as they also grow much of their own food. I know other people who consider it a major disaster if they don't have a high-speed data connection and drive-through fast food.
I'm one of those who can't make those rural living sacrifices. Rather I live in the old downtown of a small city to have most things I need within walking distance or by short car trips. My small city choice means lowest rents are $800-1000, cheapest usable houses $50,000-70,000, property taxes $2000-3000 a year, water and waste collection about $45 a month, electricity and gas about $200 (when one is high the other is low). The big expenses that don't vary much by location are communications services (entirely optional) and Medicare and supplement premiums.
Except for road trips, I drive less than 1000 miles a year. I noticed today that I put gas in my car three times in 2015, twice in 2016, three times in 2017. That's typically 6 to 8 gallons a fill, it is a small car. I stay busy volunteering in community programs, and I can usually walk to those job. Walking everywhere helps use up my time and it feels good. If in more of a hurry, I might ride my bicycle.
If you are thinking about retiring in a RV to be moving around, the moving around part is what will be what is most expensive. But you are also going to have to park a RV somewhere, and you might be looking at $400-2000 a month, depending on land values where you are and seasonal demand.
My retirement luxury is travel, whether by road trip, cruises or group travel. I'm currently far away from family, my kids move around a lot (school, jobs, military service). I usually road trip to visit family. Each 2500-3000 mile round trip to visit my brother, my sisters, or daughters uses $400-500 worth of fuel and about $200 for meals and accommodations by truck or van. If I take the RV is is more like $800-1200 worth of fuel and $150-200 for overnight accommodations enroute, another $400-$1200 for camping fees at destinations, depending on how long I stay and which place I go. If I can get bargain air fares, air travel is cheaper for these family visits.
The group tours are another matter. One to two week motorcoach tours have been $1500 to $4500, depending on destinations and whether or not air travel is involved, going much higher for destinations like Asia, Australia or Africa. I've put almost $10,000 into two to three week cruises with extension tours on each end. But I set a travel budget for each year, and it might go for one big trip, a couple medium size trips, or a half dozen road trips.
So I am suggesting, when planning for a retirement lifestyle with a limited budget, don't necessarily think of it as a RV lifestyle. That's one option, but travel is sort of integrated into that, and travel is what will be expensive. Look at low cost ways to live, and if what you want to do is RV, then budget that as a luxury paid with what is left over after living costs.
That being said, there a many people here who live full time in RVs, at low cost, by doing it with minimal travel. My cousin makes two trips a year, one from his Northern Michigan forest cabin to his RV location in South Florida, the other from Florida back to Michigan. He makes these trips slowly, visiting people and places enroute, not hauling around the RV, because his luxury is riding his motorcycles, and when making this migration he camps out of the motorcycle trailer.
