Freightliner did not plumb air for the toad until 2021 model years. Typically the “Low Air for Tow” will go off after tanks are aired up. Are you running an air line to your tow for braking? If you are not I would contact Freightliner and see what they suggest. If you are running an airline I would check my tow hook up for an air leak.
Gary and Dayna
2021 DSDP 4369 Freightliner
2012 JEEP Liberty 4WD,Demco Dominator, AF1