Remove the Cord.  Install a twist lock connector in the door opening and on the end of the cord.
I completely removed the interior uninsulated, sheetmetal cord box.  More inside storage and warmer.
I choose to use a 30A, 125VAC, 2 pole, 3 wire 
Hubbell HBL2615SW male inlet receptacle mounted on a piece of 3/8" ABS in the existing opening as seen below (NEMA L5-30).  A HBL 2613SW connector body was attached to the end of the cord.  Connector body on the cord shields the contacts so you can't shock yourself if you have plugged in the other end of the cord.  Provides a watertight connection.  

The ABS mounting plate on the inside was 2" larger than the opening on the inside wall and sealed.

Elected to insulate the backside of the plate and used the cord anchor with another piece of ABS to hold the insulation in place.

More inside storage and warmer inside compartment.  
You can also see the city water inlet has a 3-way valve used to isolate the inside water system from the outside during the winter.  Blow out the water on this short leg so it there is nothing there to freeze.  Do the same for the outside shower supply.  Avoids broken parts that you have to replace in spring.  We use full utilities during winter camping.