Pretty darn impressive numbers.
Your main losses are aerodynamics, weight, and parasitic.
-Hybrid systems are often great for cutting parasitic losses, because they usually have infinite control of the rev range and can in many cases toggle the engine on and off as needed. With a quick search I wasn't able to figure out what kind of gearbox the Tahoe uses - is this something you know and can share? Is it a planetary gearbox like the Prius/Rav4/Volt? Anyway, given the great economy you already have, there probably isn't as much low-hanging fruit here.
-Rolling resistance is affected by vehicle weight and tire choice. It tends to dominate at low speeds but is likely less of a factor on the highway. However, anything you can remove when you're not using it (e.g. 3rd row seats) will help.
-Regarding aero, I can think of a few things.
*Mirror removal is highly effective because it cuts both frontal area and Cd, which are multiplied together in the drag equation. Many people use interior mirrors, panoramic central rear-view mirrors, and a few use camera replacements. Cameras like this are standard on the new Honda-e and were one of the drag-reducing tricks for the VW-XL1.
*A partial grille block is one of the easiest ways to cut Cd slightly. Some vehicles produced in the last ~5 years have active grille shutters, which remain closed until the engine is warm, and then only open enough to maintain temperature. There are some posts about active shutters on here, but a quick'n easy solution is to just put some coroplast across part of the grille, and either add or subtract until you find the right amount, which doesn't cause heat issues underhood but also doesn't let more air into the very aerodynamically dirty engine bay than necessary.
Smooth wheel covers help a ton. (image is from my velomobile wheels)
Smooth underbody paneling is becoming more and more common from the factory, but often these panels are pulled off when a vehicle is in for service, and discarded. Back in 2008, they weren't yet very common, and even vehicles that have them tend to have incomplete paneling. Coroplast is effective and cheap.
