According to the newest version of the TM for ECV/REV trucks, DEXVI is used in all systems in the truck.
However:
GM put out a bulletin not long after they put out DEXVI
Bulletin clearly said only to use DEXVI in automatic transmissions.
It also clearly stated that other applications where DEXIII was used were NOT to be filled with DEXVI, and then gave alternative fluid labels (which were basically just repackaged DEXIII)
I couldn't find the bulletin, but apparently the below is from a GM owner's manual
This information applies to 2007 and prior GM passenger cars and trucks.
Dexron III is being phased out as a product name. It is being renamed Manual Transmission Fluid and carries p/n 88861800 U.S. (88861801 Canada).
If fluid p/n 88861800 U.S. (88861801 Canada) is not yet available when servicing a manual transmission or transfer case, Dexron III can be used in its place.
The following diagnosis might be helpful if the vehicle exhibits the symptom(s) described in this PI.
Condition/Concern:
Unavailability of manual transmission fluid part number 88861800 U.S. (88861801 Canada).
Recommendation/Instructio ns:
If fluid part number 88861800 U.S. (88861801 Canada) is unavailable when servicing a manual transmission or transfer case, Dexron III can be used in it's place. DO NOT use Dexron VI in place of the manual transmission fluid in any manual transmissions or transfer cases as a failure may result.
Please follow this diagnostic or repair process thoroughly and complete each step. If the condition exhibited is resolved without completing every step, the remaining steps do not need to be performed.