Battery Disconnect is USUALLY only related to your Coach/House Battery. It's there to keep things that are on full time, such as LPG and CO detectors, from running that battery down in storage. At least on our coach, the House Batter CAN be Off while the shore tie is plugged in or generator is running. Is this "Brake" light a warning light on your instrument panel? Same as the light that stays on till you release the parking brake? On some vehicles, there's a Front/Rear equalizing valve, and that light comes on if a leak or something else has caused unequal pressures between front and rear.
Have you plugged something in to your OBD2 test port, or one of the "Power Outlets" and left it in?