[aerohead]

I did a back-of-the-envelope calculation on the Tahoe, and relying strictly on aero, I don't see how you could have any probability of seeing above 36.6-mpg.
That's at Cd 0.12.
Without adding any length, and considering all the usual suspects, you might get to Cd 0.265.
Below that, you'd need elongation. Enough to get your wake down to about 14.5 square-feet, from about 32.0.
The 'transition zone', building into the ultimate slope angle would require 64.5-inches by itself, putting you at 266-inches, total length, from 202".
The Tahoe's squareback is your enemy. There's just no facility for meaningful drag reduction without boat-tailing, top, sides, and diffuser ( once you get one ).
You could probably pull it off with $800.00. Lots of time! Man-hours.
And then, dealing with over five feet of tail.