welcome to the forum
The GFCI outlet is part of the 110VAC side of the electrical system and has zero to do with your issue. All the circuits not working are 12VDC.
Start at the battery. What condition is it in? have you tested it for voltage/charge? if it hasnt been replaced, now wpoould be the time to do so. does everything work if your plugged into shore power? if so then we need to look at the battery cutoff and maybe the reverse polarity fuses. at this point please answer:
ARE YOU PLUGGED INTO SHORE POWER?
HAVE YOU TESTED THE BATTERY?
HAVE YOU CHECKED THE BATTERY CUT OFF?
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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.