You can never predict the unexpected costs. Those unexpected costs are something that can strand you and your coach at a distant location with no recourse other than to do what you can to pay for the repairs or sell the coach in it's unrepaired condition.
I would guess that to travel in a Class A RV you should have access to a $10,000 cushion of funds that are set aside just for emergencies. Obviously when you are not traveling, that same cushion could be used for any family emergency.
Tire can run several thousand but they are something you can somewhat plan on in advance. Other yearly scheduled maintenence items should be around $500/year or less if you do the work yourself.