where to replace popup trailer tires?

I buy mine at Discount tire. The first set of tires they replaced on my pup were underrated for the weight of our pup (3300 lbs loaded). I didn't notice the discrepancy until the first trip towing the camper. They did NOT handle well (understatement!). Discount tire happily put on different tires rated for the camper weight. Plus, I bought the incorrect ones for a different trailer- at a discount.