Ya' know, I've been noticing the exact same thing. We are in the final stages of our search for a short class A, and with very few exceptions, all of the tires have been original equipment or very near to that. But then again, when you think about it, how many miles are on those coaches we've been looking at? Compare that to your average sedan or pickup truck of the same vintage. Those folks parting with the "middle aged" motor homes just didn't use them that often.
The unit at the top of our list is a 2007 model, with just under 17K miles (and OE tires). The inspection/registration expired in 10/2015. To me, that kind of usage explains the lack of tire replacement.