I've had Bridgestone, Nexan, and now Dextero 16in. 120 load range tires. The rear tires wear out when hauling the 5er. Rotation is important, IF you want to replace tires 4 at a time. I no longer rotate my tires, I just replace the rears.
You may not find a 20inch tire in the Nexan and Dextero brands but those tires have given me good mileage and lasted longer than the Bridgestones that came on the truck.
With that size tire I would put the cheapest brand I could find on the rear. Make sure it has the correct load range and see how long they last.
On edit: Nexan does make a 265/60/R20 115S for $135.