The way I see it is you only need to look at the costs of your RV site. Stay at a cheaper RV park for a few hundred a month? Or at a higher class park for much more. Once decided that will be a fixed cost.
All other costs is up to you, and is money you would spend whether in an RV or house. Things like food, entertainment, and such depends entirely on you as to what you are satisfied with. Some can live on very little. Others must go to expensive restaurants a few nights per week and entertainment regularly.
There would be some costs to maintain your RV. But you would have most of that whether you full timed or not unless you intended to travel a lot of miles during that year. Then again, a variable due to your desires.