Perhaps this picture will help .
Do you have a voltmeter?
If hooked up correctly testing across the main ground to the main power cable should show 12.6v with charged batteries.
Many converter/chargers have reverse polarity protection fuses or circuit breakers so that would be the next thing to check.
99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.