It will cost all of your available income. ;)
Here's how I do our budget: I added all of our monthly incomes, then subtracted monthly bills like cell phone, insurance, etc. Took the sum that remained and divided it by 30. This figure is what I call our daily budget, this is the money we have to spend for RV parks, diesel, food, and all the day to day expenses. It seems to work better for me to just remember not to exceed that average amount over the course of a trip.
We also have a rainy day fund in case the bus needs emergency repairs. You really should have a monetary cushion, although a lot of people don't.