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!
I leave mine on 25 most of the time. If I'm a=on an un-naturally bumpy highway, I'll roll it back to 20-22 or if I'm in a very relaxed mood. 90% of the time I'm looking for a bit o' sport so I leave it dialed up.
On the other end of the spectrum, I run mine at 0 to 5. I weigh 145#, and have 30" legs, it does make a difference on the height of the bike, the low end, especially 0 makes the bike easier to handle, and the ride seems fine. Riding 2 up I might get it up to 10. But I am glad I can crank it up to 25, which I do every time I want to put it on the center stand, makes a really big difference.