My neighbor/friend just bought a 20-21' travel trailer. He an his wife did a 3 day/night at a local CG to check everything out. They both noticed the instability, and asked what they could do to minimize that wobbling. I have a few ideas, but felt some single axle owners may provide suggestions.
2012 Fun Finder X-139 "Boondock Style" (axle-flipped and extra insulation) 2013 Toyota Tacoma Off-Road (semi-beefy tires and components) Our trips -- pix and text About our trailer "A journey of a thousand miles begins with a single list."
If it was while towing, he may have too much weight on the rear of the trailer and not enough tongue weight. Rear storage trays or bike carriers can cause that. Additionally, enough stuff inside the trailer toward the rear can also cause sway. Move the weight forward.