Maximum tow speed for 6500 lbs Toy Hauler with two axles

Used to travel a stretch of I-80 in Ohio from Cleveland to Toledo where trucks had a speed limit of 55 and cars could go 65. Seemed to me like it was an accident looking for a place to happen because vehicles were constantly pulling out to pass; you couldn't just find a "groove" and stay there unless you wanted to putter along at 55 with the trucks.