Just like DustyDog; I too carry a tourqe wrench in the boot while towing. I also carry a list of torque wrench setting ratings for important bolts on the car and caravan (including tow ball). Thats just as important as roadwheel nuts! You don't want that to come loose now do you?
I carry a torque wrench as well but also have a line across the wheel bolts so that I can see if there is any movement. The trouble with the line though is if the wheel is removed then the lines never fit again so you have to go over them with another colour.
In the good old days when I drove an artic I had a toffee hammer. Before each trip or trailer change I would tap the wheel nuts. I could soon find a loose one by the duller sound.
I think VOSA use this method when doing roadside checks on lorries.