2008 Nimbus 338 ... Cummins ISL 425.. will only start using boost switch. Without switch on, no power when ignition is turned on. Brand new batteries for both coach and house lady week. They all have 12.5 plus on them. Battery Isolation switches are as they should be... ???
Start with the obvious - are the chassis (engine) batteries wired properly and are all connections to them clean and tight?
So... techs had dash apart, engine bay open. Bed up, three of them with multi meters checking relays, solenoids, etc... and guess what ... bad battery cable connection . wow... thank you Michelle. Life goes on...