Oh, Boy!!!
What Chassis do you have? I thought 'most every chassis had gone to 16" wheels by then. What's the background of this coach? I hope somebody didn't back-fit it with wheels/tires they had on hand or found a cheap deal on.
Fords later than 1984 will just accept the 16" wheels in place of the 16.5's. If I recall, the GM chassis (Chevy/GMC) would also. I saw a Dodge where it seemed the wheels had been swapped to 16" but later heard there was more to it. Like Fords 1983 and older...
The 8.75R16.5 size is more available today than the 8.00 and usually seems to work.
Way Way Down in
This PDF there's a Dual Wheel Spacing Chart that says the 8.75 needs just under an inch (0.9") more spacing than the 8.00. Chart explains how to determine what your spacing is now. Knowing that, you can check the offset of wheels you find.
I have the Chart as a one-page .pdf If you want it, PM me with your email ID and I can attach a copy. Used to be available on WEB as just a page but I haven't been able to find it lately.