If you haven't had the front suspension and steering of the Tow Vehicle inspected recently, I would also have that looked at. When we first started towing our Rockwood Hybrid TT (19' closed and about 5,000 lbs loaded), we towed with our Astro Mini Van. It was right at the limit of towing, but it towed well when the steering was tight and the ball joints were tight. The Astro would eat idler arms very easily and when there was play in the idler arms it made a noticeable difference in towing.
When we switched to using our Nissan Xterra for towing, it was like night and day. The Xterra did not have as much power, but the front end and steering was much stronger and held together much better. We used the same Equal-i-zer hitch on both Tow Vehicles. The Xterra towed so well, if it had more power, I wouldn't even notice the HTT was there. After a few years, the ball joints started to loosen up and it did not tow as well until they were replaced.
-Michael