The service interval for the 6L90E is 45,000 miles. I have a 2017 3500HD Gasser.
It has no drain plug and you need a fluid exchange as you can't drain the converter unless you disconnect a cooler line and pump it. (don't flush it)
I did put a PML deep pan on mine. It has a drain plug and holds 3 extra quarts. I changed the filter with the new pan. I then drained and filled it a few times. It is just about all new fluid. No need for the exchange now. I will keep it clean.
http://www.american-automobile.com/2016_silverado.pdf
Page 467 speaks directly to the service interval of the 6L80E/6L90E. Do not skip the maintenance on these. It is not listed in Normal service only Severe as that is the requirement for the 80E/90E.
The rear diff has a drain plug, I always pull the cover on the first one to clean it out. The drain plug is on the bottom of the Diff, has a fill plug too.
You can do a drain and fill if that is all you feel like.
The transfer case takes Dex VI, comes right out. Has a drain and fill plug. You will need a pump to get it back in. DEX VI is the reason you get the fluid changes more frequently. I used this pump.
Front and rear diff is 75W-90.
I am at 36,000 miles and have swapped all the fluids out.
I am in western Washington.
PML Deep pain with drain plug. You can use the stock GM filter with it. Drain plug is magnetic.