The question asked by the OP, is a good one but highly individualized as many of the above posts have mentioned. I don't know of any full timers that have over budgeted the costs, but have sure met many that have under budgeted.
So much depends on the style, the reason, etc. why a person is RVing. My wife and I did the "full time" routine for about 2 1/2 years in the Class A we had at that time, and found we didn't like doing it. We were in Florida, on the Gulf coast the morning we reached the decision to buy a stick house again. The part of Rving we so love is the travel, moving down the road, seeing new scenery, but we found the longer we were full timing it, the less miles a year we actually traveled. The last year on the road, we only racked up about 5,200 miles on the Moho. Since moving back to a stick house, we have been running between 15,000 to 20,000 miles a year on our RVs. So we are happy again, and spend about 125 to 150 nights a year sleeping in our RVs, while traveling.
The definition of what is a full timer is all over the board, it seems. Some full timers spend 365 nights a year in their RVs and it is their only residence. Others will own a stick house of some sort, perhaps a mobile home, private property with buildings, and they too consider themselves to be full timers. So while anyone can consider themselves to be a full timer, sometimes it is not easy to figure out where they fit into the full timer routine, in each of our opinions. Almost like the question of how far is up?
The advice to try it first, to me, is very good advice. Too many people jump into full timing, before they know if it is for them or not. If they have a good exit strategy, then normally not too big a problem, but we have met folks that sold their homes, bought an expensive motorhome on the credit and took off down the road, only to find they couldn't afford the life style, or just didn't like it. Some of those we met were stuck in some campground and couldn't afford to travel, some hadn't moved their diesel pushers in years, etc.
Try to figure out your style of travel in an RV, moving weekly, or staying monthly or moving twice a year. If a person can afford it, like $4 Dennis, (you will never hear the end of that smart Axxx comment) then your choice really doesn't matter so long as the purchaser likes it. But if you are only going to move twice a year from the north to the south, then why spend upwards of half a million dollars for a big DP, when for $100,000 will get a very nice large 5th wheel and tow vehicle. (I hate to see beautiful DPs just setting, grass growing between the tires, etc. when they could be out running the highways, which IMHO, is what they are designed to do)