Trans fluid runs thru the Rad 1st, then out front to the aux cooler, it's all like one big heat sink. Higher trans fluid temps add to the heat load and can affect ECT.
I'm familiar with that SB run to SA. From your description;
cruising at 60 to 65 mph and the climb was just the slight terrain incline when it seems to struggle in power even though I keep the pedal down, it slowed to 50 to 55 mph and then begins to over-heat.
60-65, no T/H engaged, running in 6th gear, @ probably 1700-1800Rs, with an 8-ton building, into a headwind, lugging it down even further as you drop to 50.
Add in the brutal Texas summer heat, That's covering everything you don't want to do.
You're just wasting fuel and making heat (and making road rage behind ya). Try this instead;
Run in M5, lock out 6th.
Keep your RPMs in the range of 2050-2200
That's going to put you closer to 67-68mph, TC locked (cooler trans), pushing a lot more air/higher boost, more coolant flow, and even though you're running higher Rs, you'll actually be using less fuel. And that loss of power thing, gone. You'll see that the throttle at that point is a bit 'touchy', enticing you towards 72mph (which your trailer tires might not like).
Also, your from a stop acceleration to hwy speed, running in T/H, you'll see that the Ally will also move the shift points up to that same RPM range. That's where the powa is.
2007 LBZ 3500 LT3 DRW Crew Cab, Sulastics, Putnam XDR 15K, B&W 30K-Turnover GN+Companion, Ride-Rite Air...EFI'd by Rob
2008 LMM 3500WT DRW Crew Cab, 8 'Flat/GN, Workforce Alum Toolbox, Ride-Rite Air Use-ta Haves 2006-6.0 3500 WT DRW ExCab Flatbed, 2005-LLY 3500 LS DRW Crew Cab, 2002-LB7 3500 LS DRW Crew Cab I Live my life in such a way that when my feet hit the floor in the morning...Satan shudders & says "Oh shit, he's awake!"