When the RV chassis/ground is allowed to float (ie like others said, no ground conductor to Earth) and while 120 VAC Hot and Neutral are connected, the chassis/ground will attempt to establish itself 1/2 way between Hot and Neutral due to capacitative coupling between the Hot, Ground and Neutral circuits.
This leaky capacitative current path is enough to feel a tingle when you made a conductive path from chassis to Earth but not enough to effect a serious shock. It is a friendly warning of a serious faulty ground condition in that your first level of safety is compromised.