I've tried Wi-Fi at all truck stop chains, and in my experience the best by far is the deal offered by Flying J. I've set up an monthly account and they bill me for around 20 bucks... I can use their Internet not only at Flying J truck stops but others that they have an agreement with (pick up a copy of the Wi-Fi booklet at a Flying J, this will tell you location and name of all their Internet access point).

A good thing with Flying J is:

a) the signal strength is usually pretty decent
b) you can be on the Internet either via Wi-Fi or hook up with a USB cable to one of the high-speed Internet sockets inside the truck stop - it's all the same money; the USB connection is often faster...
c) they can bill you monthly so you don't have to worry about your subscription expiring.

One thing though ... make sure you position your truck in such a way that you actually SEE the Wi-Fi antennae on the roof of the main building. When "you have a visual" your signal strength will be at its best.