Probability Density
You mentioned that the cities tend to keep within a "home range", if you will, of a defined size (up to the area of Mexico?) and defined shape (commonly trapezoidal?).
Even if the city doesn't follow a regular route, it stands to reason that there will be areas and locations within this range which it visits more commonly and/or at which it remains for longer. I would expect these to include points or areas in which they can harvest resources, make trades, replenish supplies (water? fuel?) and perform maintenance.
If these cities have existed for long enough that they've pretty much established their home range and where they want to be within it, then mapmakers should be able to gather the information about where they've been and extract a pattern of "probability density": a graphical representation of where the city is most likely to be found.
I would imagine two ways it could be represented on a static, paper map:
1. As variably-shaded colour: similarly to how countries are represented on a political map, you could have differently-coloured shapes on a map that define the usual home ranges of the corresponding cities. 783
Where the city spends more time/visits more frequently, the shade of the color is darker; in the parts of the range it rarely visits, the shade is lighter/more washed out.
2. As sampled past locations: the map contains a statistically significant number of past locations of the city marked as single dots of the appropriate colour. The probability of the city being in a certain area is expressed by how densely the dots in that area are clustered. (If there is a particular, exact location that the city often occupies, the mapmaker can cluster multiple marks immediately around it instead of stacking them on top of each other, since this would better express the frequency to the reader.)
Using this information, a reader of the map can hopefully make some informed guesses about where the city is most likely to be, and then seek news or rumour to confirm one or the other (if no instant communication is available). 675