The problem is probably the same relay/solenoid used for the AUX Start circuit. I have the tech info for my coach, Intellitec Mini Gas BCC. Don't know what model Coachman used but many builders used that same hardware. I'll send a PM with the PDF I have.
What the document states concerning engine charging the chassis batteries:
A. Auxiliary battery does not charge.
1. The isolator relay may not be closing. Operate the engine at a high idle for at least twenty (20) seconds and check the chassis battery voltage. The voltage must be at least 13.3 volts before
the isolator activates. Check the alternator, if the voltage is less than 13.3 volts.
Check for voltage on the coil terminal of the isolator relay. This voltage is available on J1-5. If there is voltage on the coil, replace the printed board. If there is voltage on the coil, check for
voltage between the main and auxiliary batteries. If the voltage is more than 0.1 volts replace the relay.
Not knowing which solenoid is which makes troubleshooting a bit difficult. If the isolator solenoid coil is good you can hear it click when the AUX Start switch is activated.
2006 Hurricane 31D built on a 2006 Ford F53