The viscosity retention and oxidation resistance of DEX VI are about 2x those of the fluid it replaced (DEX IIIh); that's a major improvement. While the viscosity does start lower than that of a (former) DEX III product, it actually retains the viscosity better, and after some accumulation of mileage, will be more viscus than the DEX III after it breaks down.
While it might be quesitonable for some, I see no reason whatsoever to not use this stuff to it's full capability. Dumping it early is akin to dumping engine oil too early; it's a waste.
Especiallly in the 6L80E, which was specifically designed with the DEX VI fluid in mind when it was developed.
Is DEX VI as good as top end PAO products? No - not likely. But it is world's ahead of the stuff it replaced, in some situations. It is NOT reasonably suggested to be used in older Allison 1000 series trannies in the HD trucks, to the contradiction of GMs "official" statement; chemistry creates problems for the pump seals. This information comes (quietly) from a source inside Allison. There are serial number cut-off points for where Allison suggests not using DEX VI before they made seal material changes to address the compatibility issue. It became a real he-said/she-said thing when GM owned Allison. Now, Allison is a bit more open about disagreeing with GM.
However, for most of GMs products, DEX VI is a step forward. When it first came out, it was hard to find VI anywhere but at the GM stealership, and it was pricey! Now, you can find it everywhere for a reasonable price. Licensed stuff is even available in ST-brand at WallyWorld now.
It's not the "perfect" fluid; nothing is. But it's a decent fluid that you shouldn't dump in 25k miles either; it will do a fine job in many situations and last much longer than the (former) DEX III.
In your specific applicaitons, going around 40k miles on DEX VI is perfectly doable, and you'd still have life left in the fluid when you dump it. Short trips mean nothing to tranny fluids, overall. Even towing with the truck, you should easily get 50k miles from it. If you REALLY want to know how it's doing, get a UOA. I think you'd be suprised how well it performs.