I've purchased tires at Costco and Les Schwab, both are good places but for an RV you're best off going to Les Schwab as I don't think that Costco can handle RV's. Les Schwab carries the Toyo brand which is a high quality tire, and their stores emphasize customer service no matter where located (ID, WA, OR, MT, etc). Check them out and you should be able to have tires in a day if they don't have your size in stock. And their service doesn't end with the purchase of your new tires, you get rotation, pressure checks, etc in the future at no cost; Toyo Open Country is a fine tire that can carry the weight of an RV.
