There is not enough info to make a good comment. Of course there were the normal "you need a diesel" shots.
What weight distribution and anti sway system is being used. Who set it up and have ou checked the setup?
What does the trailer weigh? Need to do all the weights as described in this forum so you know the trailer weight and tongue weight.
What were the mountains and altitude you were towing through?
What speed were you trying to maintain?
What engine RPMs were you running at
Were you using tow/haul?
Towing up hills the engine will have to work. It is not on grocery getter duty. But it does not hurt the engine to let it rev. 4000 rpm plus is just fine.
If you can answer some of these questions you may get better answers than get a diesel.