I have and occasionally use a cheap Navman F20 which seems to work very well in direction finding.
There are units available that are recommended for HGV usage, but these tend to be in the higher price scale.
If you think that the unit which you have is not doing the job intended. Then why not have a word with the supplier, possibly noting 'not fit for the purpose' under consumer law, and especially if the unit is providing wrong directions. Assuming that you are setting the sat-nav up correctly for the routes you are seeking (ie the shortest route, may not always be the best route for a commercial vehicle).