I think you will get a range of answers on this one. A TomTom or Garmin is going to set you back a good few notes. If you want to make your life easier and don’t mind paying out, then fair enough. I would be inclined to think that you are a driver doing a job for a company and they are not providing you with such an aid.
The conclusion that many come to is either a cheapo truck satnav off Ebay or use a car satnav, but keep your eyes pealed. It really depends what kind of work you are doing. Often drivers are retracing their steps and after a few weeks you may find it is just weight in your bag.
Plenty use google maps or one of the newer truck apps that you can pay for quite reasonably and the maps are offline so you are not using data. But that would be using your phone of course.