First you have to plug the generator into the side of the RV using the plug. Next, find the choke, and pull it out. Then push the primer button 3 to 5 times to inject a small amount of gas into the carburetor. Finally, pull the cord until it starts.
Each RV and generator is different, but most share the same principal. Locate the control panel. It may have a button for "priming" the gas -- similar to when you prime the gasoline through the engine of a lawn mower. Press the button a few times before flipping on switch.