In any temperature extreme situation you should consider PAO/Ester based fluid, in engine and transmission and diffs. Search hemi tick in the forum, overwhelmingly the best choice for a truck in this situation is Redline 5w30. It isn't hype, it is formula, it is built for the extremes with high additives like moly and ester based fluid. You can't fake your engine with a brand name, group 3 oils do not compare to esters when towing. Another option would be Motul, but this forum has had many more redline users with many testimonials with people that were in your situation. Pair it with a high flow synthetic royal purple filter. You can warp your heads with those temps quickly, I'd be careful. It has happened several times here, at those temps you can warp heads in short order.
I would look at the upgrades to counter that as well, oil pump upgrade and/or oil cooler and/or upgraded fan. Feelings on what should be happening should give way to accepting what is happening and dealing with it.
I would also slow way down on those grades, everything you do will help. If you warp your heads you will still have to do some of this stuff anyhow if you plan on towing a lot, you will just add that cost to it. good luck