I concur with Clarke. You want at least 25'. I carry a 50 amp 25' extension but have only had to use it a couple of times. Camping world sells both a 25' and a 30' 50 amp extension cord. Both are 3 wire 6 gauge and 1 wire 8 gauge.
If I were replacing my power cord I would go with at least a 40' (but then I have a longer coach).
Glendinning sells a 36' and a 40' cord they use with their power reels (50 amp plug on one end, blunt cut on the other) which is more flexible than the cords most people sell.