Tapping in to your well wiring has one big caveat. If both are simultaneously powered and it trips the breaker for the well, you will not have any water.
This is a situation where installing a separate branch circuit is the best way to proceed. The price of RV extension cord vs AWG 10 wire and the elimination of a long extension cord indicates that the wire should be pulled from the existing load center with a single pole 30 amp breaker and wired correctly to the location of where you would like the 30 amp RV receptacle.
This is not a difficult task for anyone knowledgeable with residential wiring. If you can pull the wire and support it, you will have 90 percent of the work done. If you've never established a new branch circuit and installed breakers or receptacles, I'd suggest to have an electrician do the work. A misstep may lead to considerable damage or more expense.
