Hello Nigel,
I make the assumption that you are asking about these features of Sat-Nav's to ascertain their ability to keep you to suitable roads for towing.
Sadly I am not aware of any Sat-Nav system available at the moment that uses the different vehicle settings (i.e. Car, HGV, Bike, etc) to select roads that are suitable for the type of transport chosen.
Most systems simply use the selection to adjust journey times. That is why we continue to hear of coaches and HGV's stuck in narrow lanes.
The systems do usually use the shortest/fastest settings to make adjustments to the route, but it still does not take into how suitable the route is for the type of vehicle chosen. It is generally the case that if you choose the fastest option, it will tend to keep you to major roads rather than side or minor roads.