We just got back from a week in Tahoe, with the Tahoe. 1,800 miles, average an indicated 20.2MPG with actual of 21.5MPG. This is less of a spread than the first road trip last summer. Still very respectable, as only 1,000 of this was the trip back and forth and 800 was in the mountains between Tahoe and Reno.
I have had no issues. The truck performed flawlessly, and I love the mirror temp gauge warning me of "ICE" when I was heading up the mountain. I would have forgotten, since it has been years since I drove through these temps, but there was a lot of ice and snow on this trip. I do not like the stock tires, so I will be changing them soon (36K miles right now).
I also had no issue with the battery ventilation. I cannot imagine a dirtier, messier, trashed interior than what the Tahoe looked like at the end of the trip, five blankets, five pillows, tons of water bottles, fast food wrappers, fast food droppings, etc. all over the interior. Obviously, from the mileage, there were no issues with the battery pack. I also found it did still use all-electric quite a bit, even on some pretty good grades. Also, on long uphill runs, I could see the mileage drop when the battery pack ran low. Going downhill, I loved 20 minute runs at 99 MPG...
I also noticed, for the first time (I'm a little slow, I guess) the CV mode of the transmission. Getting on the highway a few times, I saw the engine holding steady at 3,000RPM and the speedo rising from 50-ish past 80MPH. Very cool. I guess I don't typically go part throttle and hold it still, but for some reason I did and now I'm kind of addicted to repeating that. CV's are cool when they aren't connected to some anemic four-cylinder.
I really meant to take some pictures this time, but fate kept that from happening. I wanted a picture in Tahoe, with Lake Tahoe in the background and the Tahoe with Tahoe plates in the foreground...
...next time, I guess.

 

Sounds like fun. 20 minutes in fuel cut off... Instead of CV think of Electrically Variable EV. Did you notice the down hill engine rpm or was staying on the road more important?

1100 ish rpm at 50 to 60. V4 with fuel off. The variable valve timing VVT rolls back and less cold air is pumped through the catalytic converters. So they stay hot and the engine can go much further in fuel cut. The battery fills up so V4 fuel cut electric boost allows even more engine off operation as the terrain rolls up and down. It may be turning but the DIC 99 mpg shows it's off.

I got 20.5 in an Escalade between Detroit, Denver, Mt. Evans, Rocky Mtn park, Jackson Hole, Yellowstone, Sturgis, Mt. Rushmore, and home. The FE I lost going up was always paid back on the way down.

My kids have managed to block the air inlets so now I watch for it.

 

I did look down once in a while, and if I remember correctly, it was around or above 1,500RPM. I needed the engine braking, or I would have slapped it in neutral after the battery was totally topped off (I figure it had to be after 15 minutes).
Also, I did a lot of manual downshifting for some of the steep bits. M4 gave a little more drag than full auto. M3 would hold about 3K to 4K RPM while doing 60 downhill. I barely had to touch the brakes. I thought this would kill my mileage, but it did not.

 

When I went down the Big Thompson Canyon last summer I did the same thing. I used the Mx to control speed. Went almost all the way down with 99 mpg showing on the DIC. I had filled up in Littleton after my nephew's wedding and after Mt. Evans and Rocky Mtn park my next fill was in Loveland. I kept a log and a spreadsheet.

21.5 mpg in a fully loaded Escalade with Thule on the roof rack. That includes all the up and down plus few "goat path" passes too.

 

what thule box do you have? I have the X1 on my pilot but it would be too big for the slade.

 

Evois: It's a ski box I don't remember the model. I have pictures of the "Slade" the day we left on my hybrids collection with the box on it.

 

nice, we have the same color! with cashmere interior

 

I was towing my box. It is a GearSpace 34 by Let's Go Aero. It hangs off the hitch and sits in the vortex behind the Tahoe. I have no empirical evidence, as my big trips have been with it on the back, but it seems to increase highway MPG, but definitely does not cause a decrease, like one on the top would. I got this as my wife would not be able to reach one on the top, and the Tahoe didn't have a roof rack. Now that I've used it, it is the way to go! It is just so much more convenient and most people can remove and install it, without help.