Hello Roger,
I am sure you have seem my other posts on the subject of spring assistors.
In brief and for the benefit of Reg and Pat, if a cars suspension is to low with a trailer attached, then spring assistors are the LAST thing you should consider.
If a cars suspension is too low, there is a reason for it.
The first thing to check is the loading on the car,Is it overloaded? Both the trailers nose load and the load in the car should be checked.
Second thing is to get the cars suspension checked for wear or damage, Are the tyres inflated correctly?
Spring assistors are technically never necessary, they do not increase the loading capacity of the car.