Well from my experience traveling from tn. to utah with my pick up 22re with cam and header, basically fresh motor, I had some trouble with the hills out there and a head wind really killed me, I would recommend a size smaller tire than stock. I would say i had around 1500-1700 lbs. I was towing, i got the worst fuel milage I ever seen 12.7 at its worst 21 at best, maybe my set up wasn't that great or something. I really struggled on some of the hills 3rd gear pinned and 50 mph was top speed but we had close to 50-60 mph head wind. I dont seem to have those same problems around home though, and pulling close to same weight.
Maybe she would let you try pulling your trailer with it 1st before you buy to see if it is what you need.
Just my experience here, someone may have had better luck with different set ups.
I also had put on a size bigger than stock tire size just before trip and I am sure it contributed to my performance, or lack of.