Check the bell cranks as mentioned. At the same time look closely at the front crossmember. It bolts to frame rails in the same area the bell cranks are mounted. I found the bolts had worked loose on our Bounder. Moving the steering gear caused the frame rail to flex, top to bottom, instead of transmitting the movement through the bell cranks.
A rear track bar may be your only solution to being pushed around by semis. The chassis sits above the axle on leaf springs. This allows the chassis to sway side to side above the axle. A track bar mounts between the rear differential and the frame and keeps the chassis centered over the axle.