30-amp RV plugs have a single 120-volt hot pin, a flat neutral, and a round ground pin. On the other hand, large RVs usually have 50-amp service to power their more lavish setups (think double AC units, washer/dryers, etc.). 50-amp plugs have two 120-volt hot pins, a flat neutral pin, and a round ground.

What kind of extension cord do I need for a 30 amp RV?

Thats why you really need at least a 10-gauge extension cord for any 30-amp circuit. As you can see from the chart above, its rated for 30 amperes of current so it will have much less voltage drop (easily half as much as a 14 gauge cable with the same current).