Hello Robert,
As Eddie has pointed out, the current range of Sat-Navs, do not differentiate adequately between cars and lorries. Some models offer the user the choice of type of vehicle (Mine offers pedestrian, cyclist, car, and lorry) but all this does is apply different speed profiles when calculating journey time along the same route. This is why you do hear of some difficulties with large vehicles being sent down unsuitable roads.
A number of local councils are pressing the manufactures to improve the mapping information to help reduce this problem, sadly that is some way off in the future, and until then we will still be open to this short coming with the current systems - and of course it would need all the existing systems to be reprogrammed.
But all is not lost. Most Sat-Nav's allows the user to select between fastest and shortest routes. The fastest will tend to use the major roads, avoiding most of the small rural lanes, so when towing I use that setting, and so far I have only had 1 problem and I avoided that when I got there because the road was clearly signed unsuitable for heavy goods vehicles. I simply stayed on the main road a little longer and it safely re-routed me. Don't forget, before we had Sat-Navs none of the normal map books identified unsuitable roads, they were either to small to be included, or good signs at the entrance or driver sense was sufficient to avoid these difficulties.
If you want to reduce these issues, most systems will produce a route listing, and allow you to inspect the instructions before you start your journey. With this and a decent map, you should be able to predict where the Sat-Nav may begin to take you away from the main routes. Armed with this information you can add 'waypoints or Via stops, at strategic places around the difficult area and this will force the Sat-Nav to navigate around them.
I personally find that the road signs and a glance at a road map normally cater for the long distance runs between towns and cities. Sat-Nav comes into its own when having to negotiate unfamiliar routes in cities or towns, when you need to keep your eyes on the road, rather than chancing a look at map or route notes.
On the longer less demanding parts of the journey, it can help you to know where you are, which in the event of breakdown can be useful, or some will give information about speed limits, and your current speed, and some will warn as you approach speed safety cameras.
The other feature that most have is the ability to locate certain useful facilities, such a fuel stops, cash points and supermarkets. These are unusually called POI's (Points Of Interest).
Provided you understand the limitations of the units, then you can usually cope with the foibles of the current systems.
Don't forget that towing only represents a small proportion of our time behind the wheel, so the Sat-Nav really can shine for non-towing adventures, - try using the shortest route options around your nearest large town or city, it can be quite interesting and rewarding.
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