Question about wiring a breaker box

I have an Square D electrical box that I am wiring up in a cargo trailer. I believe this box is considered a sub panel. It's a 70AMP box. I am installing a 30AMP RV hookup plug on the outside of the trailer to be able to receive power. I am confused on wiring the feed into the box from the 30AMP plug. The plug is a 3 wire plug. I am only setting this up for 120V. I found a picture of someone elses online that is using a box like mine but it is being feed by a 3 wire (3 wires and bare ground) but my setup will be just 2 wires. I just don't know how to get power to both breakers without jumping the black wire from one breaker to the other since I have 4 possible wiring points in the panel.

Here is the pic of the one I found. The orange wire is the feed wire.http://i235.photobucket.com/albums/e...psad0nimp8.png