Are they the same size and brand (not all tires are exactly the same size as listed) on all four wheels?
Measure from a common center point at the front to a common point on all axles and make sure the distance is the same. On my little single axle trailer I noticed it dog leging on me and I found that one of the springs was put on backwards. The centering pin was about 1/2 inch off center. Flipped the spring around and it tracked properly.
1959 22' Caravanner
1988 R20 454 Suburban.