John & Gus -
The OEM tire size on the 4104 (and 4106/07) were 11.00x20. It's listed in the sales spec sheets. When radials came out, 12R22.5 became the OEM tire on the GM Parlor Cars.
But more importantly, you need to know that the whole powertrain was based on tires that turned 495 revs per mile.
So, regardless of whether you want to use 11R22.5, 12R22.5, 11R24.5, 295/75R22.5 or 315/80R22.5, you need to look at the manufacturer's specs for the revs per mile, as well as the load range for the appropriate weight capability for your coach, when buying tires.
Using tires that turn MORE
than 495 revs/mile will lower your top speed, as well as increase your fuel consumption somewhat.
Using tires that turn LESS
than 495 will have the opposite effect. (Might make incline starts a little more challenging, too.)
FWIW & HTH. . .