During our first camping season in 2011 we became intimately acquainted with rain; just one completely dry camping weekend between April and October and it even rained every day when we were in Florida. I try to avoid using an extention to my power cord but sometimes there's no option. When this happens I put the cable joint on a little pile of those "lego" blocks used for levelling the trailer, then put an upside down ice cream container on it, one that has notches cut in it to go over the cable either side of the connection. Rain runs off the container keeping the connection dry and because the connection is higher than the ground, if water gets on the cable it will run down hill, away from the joint. Cheap and cheerful but it does the job. I don't have a photo to show you and all my stuff is in storage so I can't go and take one now, but it shouldn't be too hard to visualise.
At home I have a 30 amp RV outlet just inside my garage's main door. The door closes onto the cable OK because there's a rubber gasket thing on the bottom edge of the door ,and having the outlet inside keeps it weather tight and secure.