My guess is it's a combination of two things. 7500lbs-31' and the small tires on the TT. Couple that with a 1/2 ton. The tires are maxed out being 205/75/15C. Total load rating is 7280lbs. If the TT is 7500 loaded and using 12% for TW that leaves 170lbs in reserve for each tire. That's assuming 12% TW. @10% there would only be 132lbs in reserve. That TT is squirming on the tires. Up grade the tires to either D or E rated and I bet it handles better.
OTOH 7500lbs and 31' is a load for an F150. I know I towed close to that. My Heartland 26LRSS was 7300lbs and 30'10" long. Even using a properly adjusted EQ 4pt sway WDH the TT still wandered. Pi$$ poor design by Heartland. I see they changed the frame design for 2014. Go figure.