Is an old Class-A affordable to drive?

I think mileage is probably the least of your concerns. It doesn't sound like you will be driving a lot so, 7 mpg vs 9-10 for a newer class A (and that's being generous - most still get 6-9)doesn't really translate into a lot of savings. There is something to be said for paying cash for an older motorhome, but things will beak and a lot of stuff on that coach (if it's original) has exceeded it's life expectancy. Fridge, AC, pumps, plumbing, roof, electrical, etc all may need some attention soon. Then there is the drivetrain. I am not trying to dissuade you, just that you need to be realistic in your expectations of a 31 year old motorhome. Think of a truck that was manufactured in 85, then think of a house that was built in 85, then combine the 2 and subject them to random earthquakes (what driving a MH feels like to the house) and consider what age and use does. As long as you are prepared to do or pay for repairs to the house and drivetrain, I would say go for it. But have it in your budget.