A couple of years ago, I installed an AC and upgraded the trailers 110V side from a 15 Amp inlet to a shiny, new stainles steel Marinco 30Amp twist-lock inlet and matching shore power cable from Ebay with with a Hubble twist lock plug on 1 end and a regular RV type male plug on the other, I even found a 30Amp GFCI breaker that fits in the Pushmatic breaker box and used that as a main breaker, going to 2 20Amp branch circuits. 1 for the A/C and the other for the rest of the trailer.
For the 2nd time since the upgrade, I plugged into a campsite's outlet box this weekend and got nothing in the trailer.
The plug on the box was a little busted up, and there where scorch marks on the 15amp plug, but it had power as evidenced by a 110V tester bulb I plugged into it.
The first time this happened was 2 years ago, when pulling into a KOA at 10 PM, and just happened to be the very first campsite I pulled into after updating the system.
This caused me to tear my breaker box apart, holding a flashlight in my teeth rechecking all of my work, untill I finaly figured out, with the use of a 3 light tester, that the campsite's box had an open ground. They let me plug into the neighbors's box and all was well, and I figured it was a nuance of the new GFCI main breaker I installed that it needed a good ground to work.
Fast forward to last Friday, when the same thing happened and sure enough I was at another box with an open ground. So I show this to the park manager using the same 3 light tester I keep in the trailer for such an event, and he fixed it on the spot. Voila, all systems where go and I am an electrical genuis with a picky GFCI breaker whose benevolent enough to help campsites fix thier ground bonding issues from coast to coast.
Why I was even kind enough to tighten the screws that held the head of the box to the post since I noted some arcing there which indicated a ground bonding issue.
Well that last part kept me up at night as the whole house of cards started to collapse and I realized that my origional theory just didnt add up.
For one, I dont know of any other GFCI breaker, let alone a Pushmatic GFCI breaker that was smart enough to care about a ground, and second, why would there be enough current going through the chassis of the park's electrical box to make it spark if there was a ground fault. "Well genius" says the little voice in my head that sounds a lot like my wife's, "that 3 light tester will tell you about all kinds of problems, but it wont tell you if ground and neutral are reversed."
Generally, neutral and ground are tied together eventually at the main breaker, so if the ground is good, it acts as the neutral path to the breaker and all looks well. Lights, fridge A/C work fine.
(Several people believe this to be "bad" for several reasons. -discuss.)
If the ground path is broken, then my trailer shows up with an open neutral, or a hot neutral open ground if the fridge is turned on.
That would also explain why both park managers mentioned that none of the other rigs ever had a problem with this box.
So last night found me again with tearing into the breaker box, this time with a headlamp, to retrace and recheck my work and all looks good, even reminding myself that Ground is bonded to the trailer frame, so no chance of screwing that up. That only leaves the fancy cable from Ebay...
I pluged it into a 15 to 30 amp adaptor, then plug that into a cheater plug (no ground) and into a 15A outlet and test the plug at the trailer side. Sure enough 110V between the hot hole and the L shaped ground Hole.
Unpluged the cable from the wall opened up the trailer side plug and find the nincompoop who assempled the cable managed to reverse the Nuetral and ground. These lugs are color coded, so that means they attatched the white wire to the green screw and the green wire to the white screw
Connecting the right wires to ther color coded lugs straitened everything out and the trailer is now using the Neutral path for, well, Neutral. The A/C runs better and I have less chance of energizing either the trailer's skin or the exposed conduit in the garage.
But I'm still keeping the 3 light tester in the trailer.
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Electrical mystery solved. 
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