Are you kidding? My modded (forged 348, cam, 42# injectors, full bolt ons, open-loop tune etc) '02 Z06 can get about 30mpg steady-state 75mph cruise on the freeway. Of course, I get 5-6mpg actually on a road course, haha. On an average street driven tank of gas I get around 20mpg with mixed driving. Hell, even pulling my trailer (yes, I pull a trailer w/ race tires/tools etc behind my Z06) I get about 25mpg on the highway. And again that is with the larger than stock injectors, a cam, an open-loop tune and no real concern for mpg. My “race car” gets far better mpg than my DD (Range Rover Sport Supercharged). LoL.
Stock C5s will easily do over 30mpg freeway, especially Targas since they are actually slightly slicker aerodynamically and the non-Z06s have the MN6 trans w/ the .5:1 6th gear (Z06 has the M12 w/ a .56:1 6th).
Anyway, you said no 'vette though since it’s a 2-seater. LS1 Fbodies will do nearly as well as the 'vette, but they are less aero and 300-500lbs heavier. I tended to average in the high teens in my '99 Z/28 but it always ran pig-rich (never got a real tune in it) and I drove mostly in-town with that car… and I drove like an 18 year old with over 300rwhp, since that’s what I was. 
I tracked a long trip once and averaged 28.8mpg doing 80-85mph most of the way.
-TJ