Scott's correct, the 50 amp systems are made up of 2 110 volt legs and the way the RV works is they put some stuff on one leg and some on the other. The dogbone you purchased probably only feeds one leg so only some of your stuff works. be careful not to overload, it's easy to do when you are using these types of things. Your extension cord should be high quality since you will probably burn it up. The wiring on these guys is a bit tricky...
Good luck, welcome to the forum!