Fuel costs are going to be the least of your worries concerning a 30 year old coach. A small light weight towable might be a better choice. Just finding someone who knows how to repair an engine that old is going to be a challenge.
If a class A or C is your plan up your spending budget to twice what you are willing to spend on the initial purchase. Save the additional money for initial repairs such as tires, batteries, brakes and such. You probably won't need that much but anything left over can be banked for future expenses.
Learn how to inspect a coach or hire someone who does. Concerning an older coach initial construction quality has some effect but owner maintenance is a major concern. Anything that has been sitting unused for the past couple of years could have hidden problems from roof leaks to frozen brakes. Don't buy sight unseen no matter how good the deal sounds.
Your plan is certainly doable but starting out with a reliable and safe to operate coach is going to have huge long term effects. Good luck.
