You went from 30 to 20 so the adapter is another suspect for the GFCI
Agree with smk that you might have pulled out the pin in the breakaway switch. This should cause a 12 amp draw on the batts and you should hear a humming noise from the magnets at each wheel. Put the pin back in (right way up) and all that should stop.
BTW avoid pulling that pin out when the 7-pin is connected as it can damage the brake controller in the truck doing that. Disconnect 7-pin before conducting test of breakaway braking.
12v from converter but nothing when on just battery suggests the "battery disconnect switch" is open.