How about just buying a 30’, 30amp RV extension cord? Pull out OE cord a couple of inches, plug the two cords together, amd then into shore power. This way, you have an extra long 30 amp cord if you ever need it.
Wondered why it took so long for someone to say this.
2 reasons :
First shorter run is better unless necessary. Why run 40 to 50 feet of power cord unless necessary.
Second my suggestion allows a Quickly connected surge protector to be housed in a secure not visible protected Location while maintaining a 20 to 25 ft run.
We had the dealer, where I bought the adapter, instal it, I kept scraping my knuckles trying to push the cord back in. They used the same cord for the job. It works great and has a light on it to show power.