To answer the question "How can neutral get hot"
IF you have a 30 amp rig the neutral carries ALL the current. does not matter if you are plugged into a 50 amp 120/240 volt outlet with a dogbone or a 30 amp 120 volt it carries ALL the current. So I might add does the hot wire.
When a 50 amp rig is plugged into a properly wired park outlet and if is is PERFETLY balanced the neutral carries nothing. else it carries the difference between L-1 and L-2 But if the park cheated and wired the 50 amp outlet with one leg only then the neutral carries the full load of BOTH legs and WILL get hot
If you plug a 30 amp rig into a 240 volt outlet meant for things like welders, dryers or air compressors.
Bad things will nappen and you did a fairly good job of describing them