Hello Eric
Try adding the sand bags but spread the weight out on the trailer frame to keep the centre of gravity low positioning as much as you can as near to the wheels / axel as possible.
Tow the trailer without the model before and after adding the weight to get an accurate comparison.
If this works have a wander around some scrap yards for some cast iron that you can bolt to the trailer (sash window weights are great for this)
If you are still getting 'bounce' I would suggest adjusting the trailer spring units (if possible as many are sealed units) or suspending the model in webbing slings.
Paul