If the dealer set up your hitch, it is probably not set properly. As mentioned, you may be tongue light. Another possible cause of the sway is how level the trailer pulls. If it is nose high, it will induce sway more easily. It should tow level or slightly nose down.
I would start by reading the owner's manual on your hitch, and set it up according to their instructions (with the trailer loaded up). Once you do that, head to the scale and have it weighed (truck by itself and with the trailer hooked up). That way you will know the tongue weight.