First things first. Either get new tires or get the one you have aired up to max pressure. I am a proponent of LT tires on all trucks. You do not need to go to an E rated tire as you will never use them to their potential. Get some D rated,your dealer may have to order them in but they are readily available.
Once you have the tires squared away go from there. Your trucks hitch is probably an inch or two higher than what your suv had so adjustment will be needed to neutralize that.
Did you tow the trailer loaded as normal? In my case if I have empty tanks I need three links, with full I go to four.
Let us know what you came up with!