I have had several blowouts and flats in the over 10 years of RVing. Roadside assistance only will change a tire not provide one. There are two basic pricing schedules for tires 1) normal pricing for a competitive situation during business hours where you can price shop 2) 8 PM on a Saturday night on a holiday weekend in the middle of nowhere
If you have a flat in the middle of nowhere you can spend up to $2000 for a single replacement tire, with mounting and balancing. That doesn't mean that you would even get an exact brand of tire I have heard of people getting Chinese knockoff tires at those prices. The other problem is actually getting someone out that can do the work. If you are a 100 miles from the nearest heavy truck service center it can be a major long wait for service. I work for a living that means usually when I am on my way back from a trip I need to get back the next day. I can't wait around for the next business day to get a tire! While I will never change a 22.5 tire by myself having a usable spare is critical. I have had people whip out their credit card and tell me that I don't need a spare. That is very true if you don't care about the cost. I do care. An ounce of prevention is worth a pound of cure. Have a spare and a roadside service plan and you can save yourself a lot of grief. I know motorhome manufacturers removed spare tires from their units because they were sued because some idiots tried to change a 19.5 or 22.5 commercial tire by themselves on the side of the road and got killed. In fact I have changed motorhome tires myself at home in a flat driveway with a floor jack and impact wrench on a sunny day! I wouldn't want to do it on the side of the road in the rain.