I know the OP asked about Class C, but I have some real world numbers for my Class B. I use an app on my Android that keeps track of all expenses related to a vehicle - purchase price, insurance, inspections, maintenance, gas, oil and I also record things like propane, new batteries, etc. I now have almost 50K miles on it since purchase new and my cost of ownership works out to be about $0.26/mile. To that I'd add an average cost per day when I'm using the RV to cover campground fees, food, and other misc expenses. Assuming $35/day for campground, $12/day for food and $25/day for misc my formula would be:
cost of an RV vacation = (miles * $0.26) + (days * ($35+$12+$25))
However, since I have yet to replace tires the cost of ownership per mile will increase slightly. On the plus side I've made some not inexpensive enhancements to my RV and the cost of them is included.