It can be so many things. Tire psi would be a good place to look. If that checked out it could be suspension related or sticky brake calipers. I doubt that you have a suspension issue or sticking calipers being that your truck is new but I guess it could be the case. It could be an alignment issue...like maybe it's just barely out of alignment which is why it could only be pulling under certain situations.
I would definitely bring the vehicle back to the dealership and let them sort through everything.
I would definitely bring the vehicle back to the dealership and let them sort through everything.