Welcome to the forum
I’ve seen the battery cut off in toy haulers located by the batteries, and occasion, inside near the monitor panel. When plugged into shore power does the monitor panel show all 4 battery lights lit? If so the converter is working. If no lights then look for the reverse polarity fuses, either on the back of a stand alone converter or off to the left of the 12V fuse row. They will be 30 or 40A
The transfer switch will only affect the 12VAC incoming source.
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2012 Copper Canyon 273FWRET being towed by a 1994 Ford F350 CC,LB,Dually diesel.
Airlift 5000 bags, Prodigy brake control, 5 gauges on the pillar.Used to tow a '97 Jayco 323RKS.
Now an RVIA registered tech. Retired from Law enforcement in 2008 after 25+ yrs.