Here is the picture you want:
Your RV breaker panel is just like your house panel. You have two hot sides, the sides of the outlet plug, and a neutral in the middle. Nothing in the RV is connected to both hot leads, so the 240 v you see in the top left corner should never happens. You just have to double check to make sure that either side to the bottom center is only 120v. If you wire it wrong and one of those 120 v meters is reading 240 v, you will blow up anything you connect to it.
Each side is alternating 120 v current, from plus 120 to minus 120. When you compare that to the center neutral at 0 volts, the difference between the two wires is just that 120 volts plus or minus.
The key to 240 volts is that the two sides are 180 degrees out of phase. When one side is at plus 120 volts the other side is at minus 120 volts, so at their peaks, the differences between the two voltages is 240 volts. You are comparing the two sides to each other, not to the neutral or ground.
