240VAC doesn't just float around.
The potential needs to find it's way back to the source. In the case of a WYE transformer you have 2 X 120V legs and a neutral.
The ground is intended for a path like a lightning strike. It is also a path of least resistance back to the source should the neutral leg fail.
I suggest you watch this video all the way through. It made a lot of things clear for me when I went to wire up my mobile home.
Note that when he demonstrates the potential on the neutral and ground legs he purposefully makes an imbalance load.
You've stated over and over that a 50amp service is 2 X 120V. It's actually a misnomer because at the pedestal it is 2 hot legs, a neutral and a ground.
RV loads can get extremely imbalanced depending on what is running off of each 120V leg. If the A/Cs are split and only one is running you could have a 15amp split right there.
I'll back up NY-Dutch on the 4-prong as you have both a neutral path and an EGC path.