Hello anthony
What is your reason for wanting to fit assistors?
I ask because provided you are no overloading the car or the caravan, assistors are not necessary. The manufacture fits springs to cope with the full working range of loads the car is designed to handle.
If your suspension is bottoming out when towing, then something is either broken or overlaoded.
Adding assistors does nothing to change the properloading margins, and although it may correct what appears to be nose up attitude for the car, they will not make any difference to weight distribution across the axles.
Assistors are fundamentally a devise to improve appearance of an outfit., but they may upset the carefully calculated spring rates for the car and deteriorate the handling under some conditions.