Sounds like your reverse-polarity fuses (those 40 amp orange ones) have been blown. The converter side of each one is tied to ground through a diode and a reverse polarity battery hookup will blow one or both to protect the converter.
When hooking up the battery in an RV, white is ground/battery negative and black is battery positive. That's not the automotive standard (black=negative, red=positive) but is in line with the A/C standard (white=neutral, black=hot) and is used because this is a mixed voltage system. The NEC talks about this. Better for a tech to treat all blacks as hot.
The converter is supplying 12VDC when its plugged in (and you would measure 12VDC on either side of the fuse) but when not plugged in to A/C you would only measure 12VDC on the battery side of the fuse because its not getting through to the distribution panel side.
2015 Rockwood Signature UltraLite 8282WS Platinum, GY Marathon LRD, TST 507RV TPMS
2005 GMC 2500HD CCSB D/A, Curt E16, Prodigy P2, Garmin RV760LMT w/BC-20 b/u cam