The longest distance between exits is 48.9 miles between Kissimmee and
Yeehaw Jct on the Florida Turnpike. You don't get such long gaps in
the desolate parts of the west because the highway is usually the only
road around, so there are tiny exits for each farm or ranch.
The longest distance in the continental US between gas stations is
apparently 179 miles on NV/OR 140 between Winnemucca NV and Lakeview
OR. There is a 240 mile gap on the Dalton highway in Alaska, and
the roads in the James Bay region of northern Quebec have a signed
381 km gap and one that says 500 km but is apparently really only
450 km.
For pictures, see:
http://www.aaroads.com/forum/index.php?PHPSESSID=c31f12e36c7f94cd1b1ab0e1b08e3a1f&topic=5335.msg309375#msg309375
The section of I-70 you read about is the longest stretch of
Interstate with no services, although it does have a few exits.