Possibilities
1: Bad pigtail as described in last post,, Try a new pigtail
2: Bad pigtail, worn out slot
3:Screw loose.. IF the plug on the RV's cord (50 amp)is not the original it will have screws inside
4: Screw loose,, IF the cord is detachable (most 50s are) In the outlet (The part that plugs into the RV's INLET)
5: Screw loose,,inlet
I will stop numbering.. Screw loose auto transfer switch if you have one
Screw loose main breaker
And finally Tripped or not properly installed main breaker (YES you can trip just one side).
To insure proper install press IN on the breaker, at the top.