The dispatch software we used, which was supposedly cutting edge, had horrible GPS that would show the roads, but only label the major ones. If we got a call we could see a star on the map, and that was it. There was no routing capability that would show you how to get to where you were to where you needed to go.
People would bring their own GPS units, but we really relied on map books for all of our calls. Even if I were to use the GPS the sequence would go something like:
1. Dispatch would dispatch us to a call, providing an address, map book page, and map book quadrant.
2. My partner would look it up in the map book while I got it on GPS.
3. We'd confirm our route and go.
The on-board GPS wasn't even part of the process, we'd just click "en route" on the touch screen MDT when we found the address and were on the way.
The truck's GPS system was mainly a way for dispatch to track us, not as a means of navigation.