A little more information is needed. What kind of RV, engine, chassis, transmission, year?
What light is showing? How did you get a J1939 error displayed? Do you have a Road Relay or some other kind of engine monitor?
J1939 is the data link protocol that allows the engine, transmission, and other electronic devices on your coach communicate. If you're getting this error, it's possible you have a disconnected cable somewhere around the engine control module, fuel pump, transmission controller, engine computer monitor, etc.
From what you said, it sounds like the engine still starts and runs.
Are the jacks down when you start your engine? ( I hate that noise)
2019 Coachmen Mirada Select 37LS