4 ply, 6 ply, 8 ply, 10 ply mean nothing now, that ply rating nonsense is about all tires having cotton cords. Modern tires are now 1, 2 or at most 3 ply, using synthetics (polyester, rayon, nylon, aramid fibers). The measurement standard has changed to Load Range. For LT tires, it is Load Range D or Load Range E. Load Range E is the heaviest tire with LT designation. Michelin's LTX tires are available to Load Range E in 16 inch wheel sizes, with AT and MS tread patterns.
Bridgestone/Firestone, Goodyear, Cooper, Michelin/BF Goodrich/Uniroyal all offer adequate LR-E tires with polyester or steel bodies, steel belts, and a variety of tread designs (rib, highway, all-season, all-terrain, even some winter treads). Most Class C motorhome chassis, and all E-450 from Ford, are delivered today with Michelin LTX MS/2 tires, Load Range E. While there are tougher good weather highway tires around (Michelin XPS, Bridgestone R-250) the LTX is adequate and a good all season compromise.
Without getting into subsidiary brands from the majors, Korean brands, or Chinese imports, there are a lot of LT tires meeting your needs. I think you are being jerked around by a tire salesman who doesn't have in stock what you need, or what gives him the best commission.
Tom Test
Itasca Spirit 29B