First a 12vdc problem i would check your red self resetting circuit breakers on the wall behind your batteries.
No DC power from battery. Good when on Shore power.
For a 5th wheel RV...
Take a multimeter. On shore power, check each post of your red 50A self resetting circuit breakers near your batteries. They should all read the same (about 12.4vdc).
Now remove shore power. Check them again.
Which ever one is NOT showing 12vdc on the post, it is bad. Replace it.
Note: charging is also possible via the 7 way cable from your tow vehicle. It could be supplying power on the same (good) side of the breaker.
Buy Bussmann 50A self resetting circuit breakers. Very reliable. You can get them at your local auto parts store.
Perfect flow chart:
https://www.montanaowners.com/forums...ad.php?t=86323