If your Nivomat load leveller is in good condition it should not sag if you have stock springs.
So if you check and find that it is dead, get a reco one . Last one I bought was about 14 years ago and then $300.
I also have have had Polyairs on and off /on and off over the years.
Earlier I did quite a lot of Mountain/rocky work and I feel they cut down articulation, even when empty.
But for towing on flat earth they should be good. Mine never gave trouble.
Regards Philip A