HELP! Reversed polarity of the trailer itself!

Denny16

The floating ground is your issue, as the neutral wire is carrying the ground connection also.  You need to wire up a special outlet from the main box, on its own breaker and connect the black and white wires as the others are, and a ground wire (green) from the ground buss in the box to the ground pin on the three pin receptacle,  Get an outdoor rated outlet box and weather cover and mount this on the side of the cabin, near the box, run wires in a conduit connected to both boxes.  This way, the trailer will get a ground connection, and this should eliminate the hot skin on the trailer.  The TaB needs to be earth grounded at the main box, to keep any eddy current below 5VAC.
cheers

dragonsdofly

@Denny16, thanks!

ScottG

Thank you for the confirmation, @dragonsdofly. All "neutral" and ground wires should be bonded together at the service entrance, so it sounds like your panel is installed correctly. 

Given the two-wire branch circuits and the history of the cabin, I still cast my vote for a reversed-polarity bootleg ground (RPBG). It may be isolated to the outlet into which you were plugging the T@B, which could explain why you have never experienced a hot skin on other metal appliances such as the ancient fridge. You didn't say if DH dissected the offending receptacle--this where the RPBG would most likely be diagnosed, not at the panel. You did mention that he ran an external ground for testing purposes. I'm not sure exactly what he connected where, but an extra ground will not necessarily negate the shock hazard from the RPBG (consistent with what your tests revealed).

Denny16's implication of the floating ground could be a potential explanation. The purpose of the safety ground (the third wire) is to is to reduce shock hazard by carrying current away from the metal skin (be it a camper, appliance, or tool) in the event that an internal short passes power to that skin. That last part is important--under normal circumstances the safety ground wire should NOT carry any significant voltage.

So, in summary... 

• An RPBG could cause a hot skin condition because it energizes the trailer's safety ground which is bonded to the chassis. 
• An internal short in the trailer could also cause a hot skin, particularly when plugged into an outlet that lacks a proper third wire safety ground. 

Short of rewiring the cabin's branch circuits, I agree with Denny16 that installing a properly wired external outlet would be the best solution. However, that will not correct whatever caused the problem in the first place, which may ultimately be a hazardous condition. I'm anxious to hear of your final resolution! 

Denny16

I agree, the cabin circuit should also be checked, and a grounded dedicated grounded outlet installed.  One of the outlets could have a hot/neutral wire reversed, wrong war round, connection.  I doubt any of the cabin pockets have a third ground pin in them, unless someone added one, and that would be easy to check.
cheers