Right! We need to know if it's the Chassis Battery (under the hood) or the Coach (aka House) Battery somewhere else. Our E450 Chassis will start after being parked for a month or more. But the House Battery will go dead in a few days if I don't turn the Battery Disconnect Switch OFF. In our coach, older but probably similar to yours, the switch is by the entry door. It operates a Latching Relay (that looks like a solenoid) close to the House Battery. Common culprits for running the House Battery down are LPGas and Carbon Monoxide Detectors. If your Radio runs on House Battery, the Display will run it down.