I have found myself towing 20K trailers at 90+. Easy to do with no cruise. Yes I use that when towing on the straight and flat. My truck gets better milage at 79 vs 65. You have to find the sweet spot for your truck. Mine is right at 1900 rpms. That means when towing, I drop to 5th at 66 mph and its just about perfect. I also try to stay with the flow of traffic, say 5 under. Speed limit is 70 all around here so it works out.
Quick story, I used to work for a company that had a 05 3500 DRW 6.0 gas and a 06 3500 DRW 4x4 LBZ. They towed the same trailers with the same weight on them. The gasser saw between 3-5 mpgs loaded while the LBZ saw 12ish. It also didnt struggle to get going or feel like there was too much weight behind it. We scalled 35K Daily. Plus a Diesel is more fun to play with, so you may spend a little more out of pocket, but the smile it will put on your face is priceless.