I had bucking with a Heartland 26LRSS and towed it with two different trucks. 1st was a 2010 F150 MaxTow 2nd was a 12 Ram 2500 CTD. Used several different types of WD's. Never got rid of the bucking/proposing. I came to the conclusion it was a bad design by Heartland. The TT had the WideTraxx axle spacing but they were placed more towards the middle. Fresh water tank was behind the axles. I had a TW of 900-950lbs. Roughly 12%. Took everything I could do to get it that high.
The Ram has a 169"wb and the F150 had a 145"wb. Neither the trucks suspensions or WB had any effect on bucking. It bucked on small bumps or large bumps. After 2-1/2 yrs and 10,000 miles I gave up and went the 5'er route. That got rid of the porposing:)
If it were as simple as a WB issue then why do the similar WB/TT combos tow nicely. I think some frame makers (Lippert) don't build them as well and some TT makers don't do their home work enough with floor plans. Our TT had a 9% TW from the factory, as a newbie that didn't ring a bell. Live and learn I guess.