The aspect that concerns me most about your observations on this 4x4 is the fact that its mud flaps were nearly dragging along the floor! Assuming that the driver was not driving that slowly because of some form of mechanical failure and that his/her outfit was just plainly unsuitable due to overloading/incorrect noseweight/fatigued tow car then I agree totally with your comments that this kind of irresponsible behaviour tars us all with the same brush.
However.... I recently had the privilege of collecting our first ever caravan, with the prospect of a 200 mile journey ahead of me. Not only did I have to come to terms with the outfits increased dimensions, turning and manoeuvering capabilities and reduced performance, but our tow car developed a mechanical problem that resulted in me having to travel at about 50-55mph on M-ways, substantially reduced acceleration and a feeling of 'Oh no! not another hill; are e going to make it' whenever we hit an uphill gradient, irrespective of angle.
One thing that that did occur to me was the fact that other motorists were obviously unaware that this was my first ever attempt at towing. Although I had attended a CC manouevering course prior to collecting my caravan, towing on the public highway is totally different to practising on private land.
As those who have just passed their driving test display a green 'L' plate, could not those new to caravanning also choose to display a similar sign? Perhaps a blue 'L' plate, to designate a new tower? If nothing else, it may give other motorists the opportunity to either give a slightly wider berth when passing, or to show a little more restraint/empathy/understanding.
I know that the Highway Code and relevant Traffic legislation make no allowance for 'new drivers' and that every road user irrespective of experience must display the same duty of care than the next driver.
I am not aware that anybody travelling at less than the speed limit could be prosecuted for either obstruction or driving without due care and attention! Indeed, a person collecting their first ever caravan and who drove at 37mph albeit not on a motorway, could argue that he/she was exercising exceptional due care and attention owing to their inexperience.
I am also surprised to learn of a 90% rule applicable to various driving tests including those of Her Majesty's constabulary. I always thought that accelerating to and maintaining a speed commensurate with the speed limit in force was 'where circumstances dictate and where it is safe to do so'. Certainly there is a rule of guidance that a driver should not exceed 9/10ths of either his/her or their vehicles capabilities. This does not however equate to 9/10ths of the speed limit.
Finally, I tow my caravan with a 12 seat Land Rover. Do you know the speed limit for my vehicle on a single carriageway national speed limit road or motorway for that matter? Do you know what my towing speed limits are on the above roads? For those motorists that get 'stuck' behind me on a single carraigeway national speed limit road, I can assure you that I am not holding you up; I am constrained by law to the speed limit (lower than you may think and certainly lower than the limit imposed for most tow vehicles)imposed on my vehicle.