The size would depend on if your current gearing is under, over, or just right. On a 40' coach, you can probably get away with any size in terms of load rating, so your only worry is diameter.
Chances are, your current wheels are "Budd" style. AKA stud centric. Make sure you get the wheels with the large, chamfered holes!
When I changed my rears this summer, I upsized from 20.00R20 to 22.4R24.5. This gained me 3% in gearing, which was much needed as I have a 4.11 rearend. If I went with 22.5R22.5 I would've lost 3%
I avoided 315/80 due to cost and availability.