I think gatorcq has the correct answer.
Your profile shows a 2001 36' Kountry Star DP. On the Freightliner chassis during this period, there was a batch of VDC's (Vehicle Data Computers) that had cold solder joints. Your symptoms of air pressure warning, etc. fit the symptoms reported from a VDC with cold solder joints.
Some owners have been able to repair them by carefully resoldering the joints.
I do not know if repairing the VDC will correct your mileage problem, but it certainly can not hurt.
Go to the link below and download the file VDU/VDC repair and it will detail the problem and the corrective actions.
VDU/VDC repair instructionsIf you want to understand how your instruments/guages/odometer work, use this link to download Freightliner service bulletin 54-12.
Freightliner service bulletinFor additional free help, call the Freightliner help desk at
1 800 FTL HELP. Have your VIN number handy.
Fred
Fred & Vicki
St. Augustine, Florida