Yes. Nothing plugged into the outlet, or the outlet or the wire feeding it will be protected at the proper level. The maximum breaker size for the outlet, wire, and anything plugged into it is 20 amps; it will only be protected at the same 30 amp level the RV outlet is. You would be better off, particularly as it is only 12 feet, to run a second wire and tie it into another 15 or 20 amp breaker. Don't forget, too, that any outlet mounted outside must be a GFCI style (this does not apply to the 30 amp RV outlet) and must have a cover over it. Another option is to use the outside outlet nearly all RV's have - plug in the RV and plug whatever you wish into an exterior outlet on the RV. That's how I run my air compressor when winterizing each year.