Allyson is correct to question the wording.
I do not know how it came about but several caravan manufactures do stipulate or clearly imply the caravan should not be towed by a commercial vehicle, and the examples given are quite typical.
It was explained that the suspension on commercial vehicles is usually much stiffer than on domestic cars, and as a result greater levels of vibration are transmitted to the caravan through the tow hitch. This type of vibration can be quite destructive, and cause excessive wear and weaken structures.
I recall that a company I think called Dixon-Bate produced a coupling for commercial vehicles that provide vibration damping.
My suspicion is that as Hilux' and other similar vehicles are becoming more popular as private transport, much like the crop of 4x4's that are available they are likely to have the compliant suspension that is not too damaging.
The question of affecting rights under warranty is a little less clear.
All sellers of new product are obliged und the Sale of Good Acts, to supply goods of merchantable quality, and to be fit for purpose. This does not change for a touring caravan purchased for a long term or permanent dwelling.
What will change is that the product will be subject to greater wear and tear, and this will be recognised by the courts if a warranty claim is contested.
Dealers may try to pre-empt the issue by warning customers that some warranty claims may be refused, but that is not a cut and dried position, because all products should have some useful (Warrantable) life even if it is foreshortened by extra usage.
Please note that the position with the manufactures guarantee is different, as the manufacture is not obliged to offer a guarantee they can refuse it or make it as conditional as they like.