electrical issue

After a camping trip, I set the Pod up where I could do routine after-trip maintenance. It is plugged into a GFI outlet off of the 200 amp breaker box. I noticed the dog sniffing at the tongue jack and jumping back...thought "dog is getting shocked". Tested with current detector and sure enough, trailer has current in the frame and connected parts.
The Pod has been plugged into this outlet before and checked for stray current with none detected. Why would there be an issue this time? The only things that have changed are a battery cut-off switch installed and an electric jack mounted. The foot of the jack is on wood blocking and the stabilizer jacks are not deployed. Does the trailer itself need to be grounded or should the circuit take care of it?

Thanks for the direction...spot on. The GFCI checked fine, but the extension cord had an open ground. Same (heavy) cord I've been using, but the ground wire has broken off at the plug. Re-did all the connections and all is well in the sparky world now.   Thanks again.

Edit. BTW, anyone reading into this thread needs to have the testers that Charlie was referring to. They can usually be purchased together at the local hardware store for ~$12. One unit tests for current, and the other tests for continuity and proper ground at all three pronged outlets. The issue with the dog getting shocked could have been a person grabbing the entrance handle while barefoot.

Poor dog!

+1 to ragadas, buy simple things to test connections. Makes life simpler or go thru life peeing on electric fences, or have Rover do it.