Gas mileage for 1991 Coachmen Royal 460 motor

On a coach that old depends on the tune of the motor too. I'd say 5 or 6 MPG in real world conditions.

Cruising easy on flat highway at 55-60 mph you should get 7 mpg. Pulling hills can drop down as low as 5.
Probably not what you want to hear but even the newer gas coaches of that size don't do too much better.
If you want good mileage buy a Prius.

Maybe 8 MPG on a good day. Too many variables to make an accurate guess.