When satellite navigation systems came on the market, it was a godsend for a lot of people. The ease with which you could navigate to your destination was such a huge improvement over trying to read a map, especially if the driver was alone in the car, or the person navigating had poor map-reading skills. It was almost miraculous.
So miraculous was it, in fact, that many people put all their faith in the devices, and followed their directions even if they flew in the face of common sense. Early sat-navs had their flaws and occasionally send people into rivers or down roads which hadn't been roads for twenty years, and you'd get the odd user who actually followed these flawed directions, never stopping to wonder if driving into a river was actually a good idea.
This problem was especially acute for drivers of abnormally-sized vehicles, such as trucks or motorhomes. Even if a sat-nav gave correct directions, it was possible that the vehicle was actually wider or taller than the road which they were being sent down. If an articulated lorry drove for five miles down a country road which suddenly became very narrow, it would be very tempting to try and squeeze through - the alternative being to try and reverse back up the road for five miles.
So it seems surprising that it took so long for devices to appear on the market which were specially designed for trucks or caravans. Considering that a high percentage of sat-navs would be used by truck-drivers, due to the sheer amount of driving they do, it seems obvious in retrospect that they would need their own custom devices.
And so manufacturers such as Snooper started producing sat-navs like the Truckmate into which you could enter the height, width, and weight of your vehicle and it would plot routes for you which avoided low bridges, narrow lanes, and weight-restricted roads. Also, because these devices were being targeted at specific users, for examble caravan and motorhome users, they could include lots of extra data which would only really be of interest to those groups, such as camping sites and motorhome stop-over sites.
Because these special sat-navs actually contain more information than normal sat-navs, it's generally the case that you can even turn the extra functionality off, allowing the device to work as a standard sat-nav for when you are not towing your caravan or driving your truck.
I can't help wondering what they'll come up with next: Sat-nav for hikers? Sat-nav for horse riders?