Describe 'bucking' and what that means to you.
Does that mean it's bouncing up and down, or pulling on the coach, or ??
I didn't get what the 'overpass' had to do with it either.
Your toad is free to do it's own thing, it's simply rolling along behind the coach. I have had problems with them pulling back and forth on the coach, like delayed following and so it would pull right or left, which is annoying cuz you have to correct with the steering wheel all the time to re-center the coach going down the road.
Insuring that all pivot points are tight and the tow-bar can't move in the hitch cured that. If you don't have a clamp for your towbar insert get one and try it. Pic below.
I even tried airing up and own on the toad tires so they steer easier or harder. The fix was having everything tight so it can't wander - so it has to follow.
The heaviest is the new Colorado, but now it's dialed in and does fine.
Hitch clamp...there are several but this one is my favorite:
Hitch tightener: no wobble, anti rattle stabilizer device for hitch accessories
Cars I've towed:
1. Jeep Wrangler
2. Scion XB - late style
3. Subaru Crosstrek
4. Chevy Colorado 4x4
The lighter the toad the easier it is to tow. When I had low profile tires on the Scion it would pull back and forth a lot - it would get traction with those tires. When I put the stock wheels back on it it towed perfectly.