it is normal for rear to sag when towing. half to 1" is optimal. but with all cars I've got, usually get up to 2" towing a car trailer and not using weight distribution hitch.
few fixes if sag is too much:
1. use a weight distribution hitch. this is best solution.
2. use airbag. decent solution, but it doesn't change the weight distribution
3. taller spring - but that means taller rear height when not towing.