CODE:
is for one feed to the distribution panel
all circuits sub from the distribution panel
How critical is this new circuit ?
you could but a box in the wall, install a Circuit breaker, feed the new outlet from the circuit breaker
the new circuit would be protected by the breaker
BUT you would still be against code
the reason forthe breaker is to protect the new wire from burning down your trailer, a GFI won't do that, and since the circuit would be connected to main shore input, any overload would have to trip the shore box breaker
your malfunctioning appliance could melt the wire and burn the TT before the 30amp shore breaker tripped
However since you have all this open
why not install a second box and a totally seperate 20amp circuit, with a 20 amp plug in in the shore power bay, to plug into the 20amp outlet at the shore pedistal
many people do this to have a secondary extra power circuit for an electric heater etc..
don't forget the breaker to protect the outlet