Gidday PS
Sounds like shagged self levelling struts to me, mate ...
... but your car looks too new to have this problem. Seems to show up at 100-200K kms.
First, check that you have SLS (canisters on the front of the strut).
Second, measure your tow ball weight (or jockey wheel weight, the latter being about 80-90% of the tow ball weight). Ideally, should be around 60-70 kgs, loaded. Redistribute load to achieve this ball weight.
Third, measure your wheel centre to top of wheel arch with nothing extra in the car and without trailer. Height should be within OEM spec .
Fourth, hitch trailer and re-measure. With trailer attached, rear suspension height should only change marginally (I.e. -10 to -15 mm). You will have to drive car and trailer a few kms to allow the SLS to pump up, if it's going to.
If you still have bad sag (saggy butt syndrome), replace the SLS struts with non-SLS units, as I had to do.
McHitch make fantastic products, IMO, but fitting one will not fix your rear suspension if it's shagged.