I don't know much about ac current. But here's what i do know. To find the amps of your 12.5 kw (12,500 watts) unit, simply divide the voltage by the watts. that's 52 amps peak amperage.
There's three wires (+ a ground) to make 220 work properly. One wire is neutral, two wires (legs) are hot. Since your geni puts out ~50 amps, there is 25 amps available on each leg. In essence, you have two generators in one, each able to put out ~ 25 amps @ 110 volts.
A 110 volt circuit is made by using the neutral wire and ONE hot wire. If you have big appliances that have large amperage draw, they should be on opposite legs. That would allow you to use both appliances at the same time.
The second 110 volt circuit is made using the same neutral wire, and THE OTHER hot wire.
If you measure with a volt meter across the neutral to either hot wire, it will read 110 volts. If you put the meter leads across the two hot wires, it will read 220.
A 220 volt circuit wired to an appliance would utilize the neutral wire and BOTH hot wires.
Hope this helps. I am not familliar with RV style plugs. I know Wal-mart in my town sells electrical connectors for RV's, so does home depot.
just a note:
This is forbidden by code i'm sure:
but my hot tub utilizes 220 volts. It's difficult to find a 220 outlet available for use at parties ect. I built a connector that allows the use of two standard 110 volt outlets to make 220 volts. To make this work, the outlets have to come from opposite legs in the fuse panel.
Quis custodiet ipsos custodes (who will watch the watchmen?)