wikipedia NEMA connectors, see the chart under "NEMA nomenclature"
If I was to rank them in order of most to least common campground connections, I'd do it like this: 5-15, TT-30, 5-20, 14-50. I don't mean to imply the 50 amp 14-50 is uncommon; it's actually fairly prevalent. But the others are even more so. Some campgrounds will charge more for a spot with the 14-50 connector because the person asking for it is likely to consume more electricity than the person using a 5-15 or TT-30. "Dogbone" adapters are available to down-convert most any combination (ie 14-50 on the RV down to TT-30 or 5-15 on the pedestal, TT-30 on the RV to 5-15 on the pedestal, etc).
The choice is up to you and depends mostly on how much power consumption capability you want to build in. From your other thread on fuses, it sounds as if even just a 15 amp (5-15 connector) might meet your need.
For cable.. there are lots of opinions. Mine is that a cheap home center extension cord is not an ideal choice because it's "junior" class cable and is easily damaged (if you read the codes printed/stamped on the jacket, you'll see SJ-something, probably SJTW, on its jacket). I prefer SOOW because the insulation is a nitrile rubber-PVC blend which is both more flexible and more durable. Many Home Depot locations sell it by the foot, but you'll probably get a better price at an electrical supply house. Both will have a cord end for whatever style plug you choose.