Agree with all the above, but one other thing to mention. Weight on the truck should be at least 25% of the trailer weight on a 5er. We tow an 11,000 lb Designer 5er with the standard factory pin box, and weight on the F350's hitch is around 2,800 lb. In 3 years of towing this particular trailer, we have never had a chucking issue. Just rides along as if it weren't there, regardless of road conditions. Maybe you should check your pin weight and see if it is too light.
2003 Ford F-350 V-10 Crew Cab 4WD Long Bed
2004 Jayco Designer Medallion 29 RLTS 5th wheel
Bill, Gayle, and Mindi (dachshund)
Retired at last !
Nothing is foolproof to a sufficiently talented fool.