I started out years ago towing our 7,000 lb trailer with my old chevy 2500 with the 350. I tow mostly in the Sierra and Rocky Mts. I hated the way I had to slow down to 45 mph going up the grades. So ...
If you're going too slow push the pedal down further. It's not uncommon when towing a heavy load up a mountain to have the throttle wide open. If you have pedal left you have no room to complain about the truck.