Next time you pass through Indiana, avoid all interstates! I'm not joking! I've been blazing the alarm for years on these forums for travelers pulling campers of any sort to avoid all Indiana interstates. It's been this way for years and years and seem to be getting worse, in spite of all the attempts at road construction. All they do is smooth the road out, which is a welcome endeavor. But 1 week later they are digging holes in it again and the patch work just sucks! Then, those expansion joints at bridges are enough to break the cabinets off the walls of your camper. Simple solution, avoid the interstates. Now you know why!
About the hard hitting bounce, I have found the only way to prevent that is to simply not tow a trailer! That is the only solution (or avoid the interstates).
You can try to do anything you want to your vehicle, but the shear dynamics of the trailer, the weight, the suspension, the tires, everything combined, and it will just simply never go away. You simply cannot get a smooth ride when driving over railroad ties. It just won't happen.
There have been some interstates I've slowed down to 20 mph and moved over to the right as far as I could because the hard hitting bounce was so bad, it almost caused me to wreck. And there are stretches of interstates in Indiana where it very common to see truck parts laying on the side of the road where things broke off the big-rigs.
Your only only options are to slow way down, or avoid the bumps. I avoid the bumps. We always take US highways in Indiana, never the Interstates. I'll drive out of my way 50 miles to avoid them.
And oh... an Edit, after reading back through this thread again.
Adding weight to the tongue will NOT, I say will NOT help that hard hitting bump and bounce and extreme porpoising. It will make it worse! I've tried.