To help understand your RV's 110/120V & 12V system and how "current" flows around, surf below picture.
Based on our RV's situation, it sounds like the 110/120V side is live. If 12V side is dead, ensure the AC/DC Convertor is plugged into its 110/120V outlet (not being funny but `sometimes` its plug can vibrate out during rough road trips), its 110/120V breaker is ON and its 12V fuses are ok. Ensure all looks good and 110/120V is going to its plug. With disconnected "onboard" 12V battery (to stop 12V back feeding), take a simple 12V Pen light (aka: 12V pen probe) and test its 12V "out put" ports (of the AC/DC Convertor). These ports might be white and black. Or, red and white. Or, black and white. Regardless, often thick 8 or 6 guage wires are connected into each output port. If NO 12V power coming out the AC/DC Convertor's 12V ports, then the AC/DC Convertor is shot. View diagram to understand "current flow" on both its 110/120V and 12V sides.
Hope this helps...