There's nothing wrong with your set up. That bouncing occurs everywhere. Indiana is horrible for this. THIS is exactly why we do not drive interstates any more, unless we have absolutely no choice! US Highways and secondary State Highways are ALWAYS much smoother, although we do have to travel through town traffic sometimes. That's till better than loosing the hitch.
We've had 3 travel trailers and 4 tow vehicles and every combination has done that. Avoid interstates as much as possible. And yes ... if you MUST drive these roads, than slow down and let the rest of the world pass you buy. The rest of the world will not pay for your broken hitch, or damaged trailer when the cabinets break off the wall! So slow down and control the bouncing, even if you have to slow down to 10 mph to get control again! I've done this many times and pi$$ed off lots of auto drivers. But they aren't paying for any damage "I" may encounter either! So as Chef Ramsey may say, tell those other drivers: "Pi$$-off!" and first exit, take secondary roads.