I think one needs to define for themselves, what they mean by fulltiming in an RV. Does your definition include workamping/volunteering X number of hours/week for X weeks, month(s) or a season in return for an RV site to keep costs down or working at Amazon, gate guarding or the sugar beet harvest part of the year to earn enough to travel the rest of the year? Do you just want to live in an RV or actually travel in one? How much and where do you want to travel? Do you want to just move from a warm winter location to a cooler summer location taking advantage of long term rates and places like the BLM LTVAs? Do you want to travel mostly from one membership park to another? Do you want to mostly boondock?
Two of the more controllable costs are fuel and camping fees. Fuel cost is pretty simply to manage. Don’t drive and you won’t need fuel. Which brings me back to the question of what do you want/plan to do in your RV, sit in a few places or travel and see the country? Many RVs get about 10MPG. So, using that and $3.00/Gal, every 1000 miles will cost $300 in fuel. Fuel prices can vary a fair amount from state to state due to the fuel taxes. Fuel prices can also fluctuate widely by time of year and other market factors.
Discussions of yearly camping costs usually result in widely varying costs ranging from a few dollars/night to $20-30/night. You need to understand just how those costs were achieved and the extra costs that might be incurred but not counted in the calculation of nightly costs. The extremely low costs often result from workamping that includes a free site; volunteering that includes a free site; staying primarily in membership parks such as Thousand Trails; extensive dry camping in Forest Service, BLM or other public campgrounds and boondocking. If one moves once a month and can get a rate of $250/month that is $3,000/year. If one were to stay less than a month and get a weekly rate of $100 that is $5,200/year. A nightly rate of $20 comes to $7,300/year. In my experience those are fairly low rates for commercial and public campgrounds. We have found that BLM, Forest Service and Corps of Engineers campgrounds often offer sites with electric service at nightly costs between $3.00-13.00 with the Senior Pass. However, they rarely offer weekly or monthly rates and often have stay limits of 14-21 days. Travelling the U.S. highways, we have found city parks (even some with hookups) that allow overnights for free or a very reasonable cost. However, they also often have stay limits, sometimes as short as a day or two. So you save on nightly fees, but use fuel to get to the next location. When travelling from destination to destination, Walmarts, truck stops and other areas that allow overnight stays can save a considerable amount on camping fees.
Without major changes in lifestyle, you will likely spend about the same on most things as you do now. What you will spend fulltiming in an RV will depend a lot on you and what you want to do.