Yes, 22,5 tires are pricey. Most companies recommend replacement every 5-7 years OR have them dismounted and interior casings checked for integrity every year thereafter. Can you run them longer? Of course. Should you? Is it wise? Depends on how lucky you feel and whether you are willing to risk your safety and/or those who share the road with you. Anyway, cost is also dependent on the size of the tire. 315s will cost more than 270s etc. FMCA has a discount program which I've used for my last two sets of Michelins. I won't skimp on tires as I feel they are probably the most important aspect of safety. Also it's no fun sitting on the side of the road waiting for the tow truck to change your tire, to say nothing of the damage done to your coach if the tire blows at 60 mph.
That said, it's IMPORTANT that you know/learn how to read the date codes on the tires. The date code will be molded into the tire casing and is usually mounted on the INSIDE so it takes a bit to locate it. Knowing this will give you info on the tire life left. Obviously, on a used coach you'll have zero knowledge as to what kind of life they've had so check them carefully. Also, don't assume because a tire is NEW that it's really new. Check date codes on those also. When buying tires, I insist on tires less than 6 months old. Finally, use the search box at the top of the page to see previous posts and obtain a lot of good information that has been previously discussed. Tons and tons of great info available, especially about tires.....Dennis