It could be that the new trailer does not have enough tongue weight. Less likely but still possible is too much tongue weight. Have you actually weighed it? Dry and brochure weights are meaningless, you need to go to a truck scale.
If your dry weight is 7,800 you are likely pushing 9,000 fully loaded. If your tongue weight percentage stays the same that will be nearly 1,200 pounds on the tongue leaving you with only about 500 pounds for passengers and whatever else you put in the truck.
You may need more air in your tires on both the truck and possible the trailer; have you checked the air pressure?
Do you now have more weight in the cab or bed of the truck then you did before? More people, loaded tool box, generator, firewood, etc?