Slower speeds will have the biggest impact on mileage. The difference in effort to maintain a given speed is exponential between say 60 and 65. You can help this some with aerodynamics but the cost of those modifications will seldom be offset by their improvements, simply not cost effective. Your best bet is to be sure your motor is tuned properly and to drive at the speed it is geared for, given the age of your coach I'd guess that speed is 55mph.
Obviously any weight reduction you can do will help as well.
