The outline of costs that Jim posted earlier in the thread is pretty close to spot on. Just a slight addition would be registration and insurance. With those added in the cost would usually increase significantly and obviously how much would depend on annual mileage.
Pre-retirement, I operated a small fleet of 33,000 GVW delivery trucks. Obviously quite similar to a motorhome in size and components. We found our fleet average cost per mile to be right at $3.00 per mile. That would vary slightly from year to year with fluctuations in fuel costs and any significant repairs to any one or more units, but it was a pretty consistent number for the 2008 thru 2011 timeframe. I don't track my coach operating costs close enough to say where those numbers would fall, but it ain't cheap.