The good news is if everything works when you hold the Aux Start switch, that says your Aux Start relay and everything beyond it is OK. The Aux Start relay only activates while you hold in the Aux Start switch, so it's operating normally. It connects the House and Chassis circuits together, letting the House batteries boost a low starting battery so you can start the engine. It also sends voltage the other way, in this case connecting the dead House circuits to the good starting battery.
This means you're losing connectivity somewhere between the House batteries and the Aux Start Relay. It may be a bad battery disconnect relay or you may have blown an in-line fuse or circuit breaker.
Start by physically tracing the wires connected to the House batteries. Start with the (+) wire and trace it towards where it connects to the Aux Start relay. A voltmeter or 12 volt probe light will make the process much easier by letting you see where the voltage disappears.
