New TV tires today

Just went through that with my 99 Ram 2500, and it sucked. I had P series tires running 55# all the way around and it handled towing great. Had new Cooper LT "E" series tires put on and aired them to the 55 front 80 rear like the door post says and couldn't keep it on the road. Kept stopping and dropping the air 5# at a time until I got it down to 60# in the rear and it handled a lot better. Still doesn't handle as good as the P series did but that might just be because they are new. Anyway, back to your question, I'd start at where you had your old tires set at and slowly increase the pressure. Of course if you want to spend the time you could always do the chalk check. Draw a line all the way across the tread on the tires and add/remove air until the line is completely gone. More air if it wears on the outside first, less if it wears in the middle first. When it wears evenly all the way across you have the tires inflated correctly for the load.