PMFJI with an unrelated topic, but… first, I really love Highway Wars; it give me the chance to act out as a “knight of the future.”
But I think you might have missed out on something: Radio.
Radio reception doesn’t necessarily need any outside power source at all (ever hear of crystal radio?), it’s only transmission that really needs external power. But any Free City able to supply power for air conditioning its hotels most certainly has plenty enough power for radio transmission. Nor is the technology very sophisticated – certainly less than would be needed to maintain an ID card reader/recorder. 
Radio would both simplify and complicate a number of things in this story; the Free Cities could more easily share their needs and expertise with one another, and they could also keep closer track of the various Highway Marshals floating about (by the bye, you misspell the word as “Marshall” very early on in the story, which always bugs me whenever I see it, but I digress) and the Marshals could more easily keep in touch, not only with the Free Cities, but also with one another. If power is at a premium, perhaps sending would be limited to emergencies only, or perhaps to certain specific times, etc. (Indeed, certain times of day would be better than others; I forget the exact details, but bouncing signals off of the ionosphere for long-distance transmission can only be done when the ionosphere is actually ionized, which it isn’t always.)
Radio would also alleviate the need to travel simply in order to convey information. For example, one reason the encounter with the man-eaters (I forget offhand what you call them) is such a dilemma is that notifying the militia of their existence would mean physically abandoning the mission in oder to tell them. With radio, the MC could simply notify the nearest Free City of the threat right on the spot, then continue his mission!
BUT: I would also envision the following complications. (1) The various warlords could use radio, too… or simply try to jam the communications between the Free Cities – and he Free Cities, of course, would reciprocate! (2) Cryptography! Advaced cryptography – the kind today used by computers – would be right out: forget AES or Twofish. But classical cryptography, of the pen-&-paper variety, would enjoy a resurgence… with cryptanalysis (“codebreaking”) following right along! In fact, this might present some opportunities – instead of an engineer, maybe the kidnapped schlub is a cryptographer with knowledge either of a more secure cipher, or the solution to a cipher used by some of the warlords. (3) Radio would allow for “silent” alarms. Instead of sentries or scouts ringing bells to notify their host cities (giving their presence away, and warning the warlords as well as the City), they could send a silent alert by radio, perhaps resulting in an invading warlord force walking into an ambush. (4) Cities (or caravans equipped with a transmitter and solar cells to power it) could send out an SOS when in trouble, bringing Highway Marshals to their aid. Etc. etc. etc.
Any thoughts?