There are some items you can get close on, once you decide how you plan to use the RV. When we were full timing it for 2 1/2 years I had our costs figured out on the fixed items.
Lots of variables, such as are you going to do your own maintenance, such as oil changes, tire rotations, etc. or take it to a shop. Do you have a place to store the RV at no cost or are you going to pay storage somewhere? Insurance is some what fixed, etc.
I remember after we came off the road and were putting about 5000 miles a year on the Class A gas unit we owned, fuel was about $.50 a mile at todays prices, oil changes were about a nickel a mile, tires about the same, storage was about $.24 a mile, tune ups were in the neighborhood of a nickel a mile. Then it comes down to what you want to spend on it, say a new under cabinet coffee pot, and that list is so personal only the owner could guess at that, etc.
Over all I used to figure on about $1 a mile above and beyond, the payment, camping fees, etc. Someone once said a boat was a hole in the water into which you poured your money. An RV could be said to be a bump on the highway into which you poured your money.
Figure out how you plan to use it, first of all. If you plan to go to the lake that is 65 miles away from your home, 8 or 10 times a year and park for a week. Then your costs will not be the same as if you decide to take a few months and drive to the 4 corners of the USA.
Buying a 5 year old unit will get rid of most of the depreciation on the unit and that is as real a cost as buying tires, in the long run. Call up your insurance agent and ask about the insurance costs, check with the tag agent to see what it will cost per year. Some costs are fixed, some are discretionary, and some are just "got ya's" that no amount of planning can get you prepared to budget for.