I think you mean 8R19.5? Which would be 19.5 inch truck rims? These are usually used on HEAVY RVs, so I'd be surprised if there were any cheap substitutes for them. These are usually 10-ply equivalents, I think. How much does your RV weigh?
I have heard that the Family Motor Coach Association (FMCA) has a sweetheart deal with Michelin, members can buy these kinds of tires at a substantial savings. Supposedly you can save WAY more than the cost of your annual membership.
Not really my area of expertise, I'm into truck campers myself . . .