Sure sounds like one leg of the 50 amp is not there or your plug or somewhere back from there is open. Such as a breaker on your own AC panel.
Cavie has it nailed. You need a voltmeter. Everyone that owns an RV should learn how to use a voltmeter with confidence. They really are quite simple, but very confusing with all those weird symbols.
I told my brother to take pictures of his as I was teaching him. He did not nor did he mark his voltmeter to remember what position to put it on. I will be putting on a class again for him and he will take pictures.