Ok, then go to a RV dealer.. My primary 30 amp extension cord has since been converted into my primary 30 amp cord.. It is plastic wrapped, not rubber, it's bright red as I recall, not black, and it's very easy to wrangle when the mercury is ... Where it's been the last few weeks, they synthetic jacket does not get "HARD" like the rubber cords do when it gets cold... What can I say.
OF course this being a 50 amp rig I have a 50 amp cord too,, both of 'em have Marinco 50 amp outlet ends.
But that said.. I will modify my suggestion.
If you want to put in a permanent parkign spot for the RV, one that you will still be using years from now. Go with 8/3 Underground feeder wire and a pedistal. YES 8/3 to cut down on the voltage drop.
If you do not plan on making that the forever site for your RV
Go with the standard 30 amp extension cord from a reliable RV source and the ones from Wal-mart should be good.. just weather resist the joint as I described above.