I am an accountant so I track every penny. I use an excel spreadsheet to forecast total costs.
I figure $1/mile (1 way) for gas and that includes the return. So if I'm planning to go 1,500 miles (furthest point), I budget $1,500 for gas. (Based upon my Lazy Daze Class C 8.5mpg)
As mentioned in earlier posts, camping fees can be quite low (on average) if you boondock some. We use WM and no hook-up sites to keep the average lower.
I recommend doing an overall personal budget/financial plan to make sure you can afford RV travel. If money becomes a stress point, it may not be any fun.
I will send my RV spreadsheet and personal budget spreadsheet to anyone who wants a copy.