This thread reminds me of when I bought a new 18' travel trailer in 1975 with tandem axles that weighed 4,500 pounds. I towed it with a 1970 Olds 88 4-dr sedan with a load distributing hitch. The RV dealer installed the hitch, hooked up the trailer, etc. The hitch head was one of those that could be raised or lowered by putting the bolts in different holes, and it could be tilted forward and back by putting washers on a bolt. When it was all done, the rig appeared to be sitting level and looked good, but when I started down the road, it felt almost like the car was surging, with the trailer jerking fore and aft. The RV dealer didn't know what to do about it, so he told me to take it Joe's Hitch in Dallas, and let them fix it and he'd pay for it. Well, the kid at Joe's Hitch went and got Joe, the old man and owner. He just glanced at it and said it needed to be raised one inch (one set of holes) and have one washer added to tilt the ball back that much; said he could do it, or take it back to the RV dealer and let them do it. I went back to the RV dealer, he did it, and problem fixed.