I agree, pick your trailer first then size the tow vehicle accordingly. You'll spend a lot more time in the trailer than the truck. That has to be happy.
I'm a diehard Ford guy, love my Superduty's. I'm on my 3rd, a 21 SCrew, long bed, long wheelbase, 6.7 diesel dually. I do not even know the 18k I tow is behind me.
BUT, the RAM is certainly an option. The Cummins is an almost bulletproof platform, and RAM has come a long way. They will still rust out faster than anything else though.
Your comment about 6 speed VS 8, you'll find diesel has so much more capability that even 6 speed is enough. My 21 has a 10 speed, my '16 only had 6. Both were (are) very good . When towing, my '21 uses all 10. Daily driving, it skips several in-between gears as it shifts, so it's a 6 speed for all practical purposes.
My good friend bought a new Chevy High Country Dually 4 door long bed with the Duramax/Allison. He tows RV's across the country from Elkhart to everywhere. It does the job very (VERY) well, with zero drama.
It's a wash. The Chevy Allison trans fluid is expensive, my Ford does fine on 17. a gallon Valvoline from WM. Chevy makes up when you change engine oil, 2.5 gallons of 5w40, where the Ford takes 14 quarts. 10k changes for the Chevy, 7.5 for the Ford because Ford injects diesel on the exhaust stroke for DPF regen and it dilutes the oil, where RAM and Chevy use an injector in the exhaust to inject for regen. Oil lasts longer.
My Ford wins in the BAF (Big a** fuel) tank though. 49 gallons from the factory. To get that in Chevy or RAM, you'll be buying an aftermarket Titan or other tank.
Try them all. When you are at that level, the differences are more personal preference than outright advantages.
And good luck finding your hardware. It's a search, but worth it in the end.