HI Tom and Welcome to the forum!
A major roof leak would be a deal breaker for me, but if you are handy and not afraid to do all the work yourself by all means go ahead. The thing that would concern me is, how long has it been leaking, how much unseen damage is there, and has it also damaged the walls and sub-floors.
As for fuel economy, not so good. Back then there was no overdrive and most chassis were built very close to their max GVWR. 5-6 MPG on a good day.
Remember, it's a buyers market. For every used coach you see that looks ok there are probably 10 fold more that are in better shape.
'98 Gulf Stream Sunsport 325, 7.5L Banks Power Pack, Koni FSD's, Air Bags, ReadyBrute Elite,
2000 Honda Accord