Re-check your hitch set up, it was not performed correctly.
http://davidsrvtips.blogspot.com/2009/02/hitch-setup_2781.html You need to have enough weight on your rear wheels while keeping your front wheels in check to allow steering control.
Just because the dealer did it does not mean it was done correctly.
A 2017 28 ft Jayco does not need sand bags.
As well as setting up my own and recently went through this with someone and a new Jayco already . I had them call the dealership from a rest area where they had to pull over. The dealer sent out a different mechanic who told him the hitch was not set up correctly. After a few adjustments, it towed like a dream. Not enough weight distribution on the rear wheels.
