First off, welcome to winnieowners.com!
Second, you should add a "signature" to your profile. It contains whatever info you want; most people put info about their coach there, so they don't have to type it into each message. Instructions for adding a signature are here
Re: 12V. The best way to track down 12V problems is with a voltmeter or 12V trouble lamp, starting at the batteries.
- Disconnect from shore power
- If your Winnie has solar, turn off the solar controller
These two steps ensure that you won't have any 12V charging current.
- Apply the voltmeter to the house batteries. Record the reading.
- Follow the positive wire lead from the house batteries to the Battery Mode Solenoid. Apply the voltmeter to the large lug to which the cable is attached. Record the reading.
- Follow the cable from that large lug to the Battery Disconnect Relay (solenoid). Apply the voltmeter to the large lug to which the cable is attached. Record the reading.
- Apply the voltmeter to the other large lug of the Battery Disconnect Relay. Have someone press the AUX switch. Record the reading.
- Have them press the AUX relay again. Record the reading.
That's enough steps for now. If you consistently get 12V through the Battery Disconnect Relay, we'll look downstream for the problem source.
Unless your Winnie has an inverter, there will be no 300A fuse.
Wiring diagrams for your coach are online, here
. The batteries, solenoid and relay show up on the "Body, 12 Volt Wiring Diagram," page 4, which is sheet 1, on zone A-12 (link here