+1 for manual downshifting. In fact, many GMC'ers disconnect the transmission kickdown switch under the dash to avoid shock to the transmission while under load.Need? No, but if it gives you peace of mind then go for it.
In reality your ears should be all the tach you need (apologies if you are hearing impaired). The trans is cooled by the same water as the engine so if your water temp gauge is normal, then your trans temp should be fine as long as your fluid is topped off.
When climbing a hill, downshift from D to S when you slow to 45. Use S or L as necessary descending grades to save the brakes.
In the past year I have been over the Siskiyous, the Cascades and the Sierras several times and my GMC instrument panel is stock. I do have a external trans cooler in addition to the in-radiator one.
Between the temp gauge, the fuel gauge and the speedometer I have enough gauges to distract me and make me nervous
I have a PO-installed finned pan on my coach as well. FWIW, some folks don't like it because of the way it protrudes from the undercarriage, increasing the risk of impact/damage.
And making sure your engine cooling system is tip-topA tranny temp gauge is a cheap and easy thing to install. Very helpful. But.... sometimes there is no place to pull over if it gets hot. So keeping it from getting hot in the first place is key.
a big 40,000 gvw tru cooler tranny cooler is not a bad idea.
i think it also depends on if you are towing anything as well.
This flat lander creeped over continental divide 8 times on my virgin trip from North West Montana back home to South Alabama. Not an expert but I was careful not to overheat my brakes On descent by down shifting. I also switched to my rear gas tank on ascent and back to front tank on descent. At least watch your fuel level. Good luck.