Welcome, my tow vehicle is Mercedes ML350 BlueTec Diesel and it handled the 19' FC easily, no worries and issues. My Benz is actually a bit shorter than yours and doesn't have the air ride, but I have not experienced any serious towing issues and I cruise at 65 MPH. I have since switched up to SOB but much larger (24') that weighs in at about 5,500 pounds with 600 on the tongue. Again, no issues and no worries. Make sure you use a proper weight distribution hitch, get a good controller and pay the dealer to install the brake controller to the stability control system, it really is worth the extra cost.
To answer your question directly, I'd stay under 25', you should be more than ok if set up properly. Good luck.
One Long Shot
Beastie - 19'FC
The Beast - Mercedes 350 BlueTec
Held together with the Husky Centerline Hitch