I thought that too until I learned how this actually works and watched some Youtube videos explaining it. Here's what actually happens like in an RV.
It is using the connection as if it's two circuits of 120V. Inside the RV's electrical panel, there is a shared neutral, but it's expecting that to be tied down to 0V to hold it in the middle. If it's not, then it's really nothing more than a pass-through wire joining the two circuits together.
I'm going to refer to "A side" and "B side" as like the two sets of 120V circuits that have the common neutral that is disconnected. You have several things like lights, power outlets and such on each of those sides. What you end up having is A side and B side connected across a total of 240V with nothing to force it to stay evenly with 120V on each side. So with random combinations of things plugged in, the resistance will be uneven, and the voltage may shift wildly, like 40V on one side and 200V on the other. Some appliances built only to accept 120V may pop or blow their power supply and start a fire if they get 200V on them that they are not prepared for.
If you search Youtube for "missing neutral", you can find some good videos on it with demonstrations.