You are testing an abnormal condition... When switch is in 'USE', coach batteries are expected to be present,,,
Not really surprised by your result... I expect the 12v chassis power would disconnect as its voltage dropped.. Actually nice that everything can still be powered (for a little while at least) on a single battery.
Certainly possible it is wired incorrectly...
Not yet retired...
Florida (Michigan transplant)
2014 Hurricane 32A
2000 Infinity (previous)