Check the alignment of the trailer axle as well. Had a small utility trailer that had a slight cant in the axle. With higher speeds, it was all over the road. There would be evidence of this with weird tire wear patterns.
Chuck and Joanne
2014 Flagstaff Super Lite Classic 27RLWS Emerald
2013 Dodge 2500 4x4 Laramie Crew
Equal-i-zer 1200/12,000 4-point WDH