[redwngr]

Fwiw, the little 10 speed behind the 3.0L is also an excellent combination.

 

[redwngr]

In manual mode, the computer will not do the automatic downshifts that it does do when in automatic mode.
Manual mode is telling the computer it's opinion is not wanted.

 

[redwngr]

They'll rev higher when doing engine braking than when producing power.

From L5P specs at GMPowertrain

Maximum Powered Speed 3450 rpm
Maximum Braking Speed 4800 rpm

 

[redwngr]

It’s interesting to me that my truck seems to be one of the only ones that has a properly working EB. I set my cruise and the truck keeps me at that speed. Heck, I can knock my speed down 10 mph with the cruise button and the truck will slow me down just fine. Not sure why you guys buy these fancy trucks engineered to tow and slow you down properly and then you throw it in manual mode thinking you know how to shift better than the truck. T/H on, EB on, set cruise, sit back and steer the truck in the right direction, its not too difficult.

That was true with the earlier EB trucks as well.

 

[redwngr]

Different doesn't mean it's wrong...
I prefer the way GM does it.

 

[redwngr]

If you know you’re towing you could always start in L and max the gear allowing to whatever you want and it’ll drive like normal. Then when you want to downshift just click the buttons as you say.
I personally only tow in 7 (1:1) or lower so I keep it in L7. If I see a hill coming or want to stop and use the exhaust brake more effectively I can click my buttons down and increase my rpm’s. Seems pretty easy to me

If you leave it in auto, the truck will downshift to increase exhaust brake effectiveness.

If you run in manual it will not -- because running in manual indicates to the control that the operator wants to manage the 'downshift for exhaust brake or powertrain grade brake' decision.