DIAGNOSIS The 50-amp (TT-30R) outlet is on a completely separate circuit from the 120V duplex outlets. A single point of failure for just that outlet is usually the GFCI device it's wired through or a loose connection behind the panel.
FIX Find the GFCI reset button. On this model, it's typically a physical button on the control panel, near the outlets, sometimes labeled "GFCI Reset." Press it firmly. You should hear a click. Then test the 50-amp outlet with a voltmeter or a known-good RV cord.
IF THAT DIDN'T WORK The GFCI receptacle itself can fail. You'd need to remove the control panel cover (disconnect spark plug wire first!) to visually inspect the wiring to the 50-amp outlet and the GFCI unit for burns or loose wires. The GFCI is a replaceable component.
HEADS UP This GFCI is super sensitive and can trip if there's any moisture in the cord or the connected load. It's a safety feature, not a defect. Always ensure your extension cord and appliance connections are dry.