I've towed with multiple gassers and multiple diesels. If he's going to be towing a big (or heavy - 8k+) trailer, he will not, IMHO, be happy with the gasser. I towed a pop-up and a small Scottie trailer with a 1/2 ton AND a 3/4 ton gasser (both Fords... sorry) and they were okay.
I then graduated to a 5700# Avion TT and it was not happy with the gassers. The diesel pulled it fine. Now, I tow a 10,500# 5'ver with my current 7. 3 Ford and it does a good job..... just hairy going down hill with an auto transmission.
When I towed the Avion (5700#) with a 3/4 ton Ford gasser, I got 8-9 MPG and I never tow over 65 on the Interstates, slower on other roads. My diesel gets 13 MPG towing 10,500# over same road... . and my 5'ver has a huge head that catches the air and hurts the MPG. I'm convinced I could get 16 MPG towing my old trailer... . about 60-70% better fuel mileage.
I often pass gassers on long grades out West with my rig. They're revving about 5k RPM and struggling on the hills. I'm out of overdrive, but cruising up the grades in 3rd gear at 55-60.
And I can keep my truck for as long as I want... . I always get tired of the diesels before they get tired. After 40 years+ towing all sorts of stuff, I believe the diesel, with a $5,000+ premium, is still the right choice.
But, I know the choclate and vanilla theory. Good luck.
Florida Ed