I believe you have 365/80/22.5 tires. Their diameter is 45.5" Tire covers are usually sold at most Trailer supply businesses. I've always purchased my covers from Camping World. I'd give them a call 1-888-626-7576 and tell them your tire size and diameter and see if they stock covers for your tires. Their covers last for years and the only time I've had to replace them is when I've purchased a new coach with different size tires.
2016 Newmar Dutch Star 4369
Newmar Owners Club
USAF 1966-1969,- Law Enforcement 1969 - 2003, Unemployed (retired) since March 2003