My 1986 Winnabago Elandan with 454, 3 speed trans and carb gets between 8.5 and 9 MPG and with the money we saved buying it We can buy enough gas to drive it for about 50 years.The question you have to ask yourself with an older motorhome is "how much of the upkeep and small repairs can you do yourself?". It will take a while to work out all the bugs and update it the way you want it. If you have to take it in and pay the ubsurd prices the dealers charge then something newer will end up cheaper. But if you can do it yourself you can make it exactly the way you want it at a huge savings.
