The 1800 does not have an air suspension!
It has a hydraulic assisted preload spring on the single rear shock. This works different than air assisted shocks!
I will suggest you put it on the upper reading of 25 and try it there for a while.
This was suggested by a very smart man on another board. Tom Finch-owner of TailWind Motorcycle Trailers.
After his explanation of the design and function, he suggested the upper setting and leave it there.
My wife and I were getting a little upset(this is all prior to triking)not finding a good setting that especially was good for her as a passenger.
I upped the preload to the max. 25, not saying anything to the wife. I was positive the first "bump" we hit, she would be banging on my helmet asking what did you do, take the shock off?
Instead, after a few miles, over the intercom, she asked what I had changed, it's riding better than it ever has!
Told her and never touched the preload buttons again with or without a passenger!