On my old 5'ver I always had to use a 30' extension to get to shore power(30 amp), I think my trailer came with like a 20' cord. Instead of trying to waterproof the plugs, I made a hanger on the frame under the trailer that I could hook the connection in. This would keep it up off the ground, and out of the rain. never had a problem with it in 5 years!
2014 Voltage v3990 - Epic Package
2005 Ford F-350 DRW With 6 Liter and 3.73 Rear End
B&W Turnover Ball with 5th Wheel Companion Hitch