Tripping breaker at the stanchion

Mystery.

Came home from a long trip where we stayed at no fewer than 8 different marinas. No unusual power issues.

Get to home marina after a 4-hour lake run, plug in shore power, and BAM..... breakers blow immediately on the pole when ANY load is placed on it. Normally I run a single 30 amp to a dual 30 Y-cord, so I pulled out the Y and....

Tried:

Running with two 30 amp cords to multiple poles.

Tried each 30 amp input by itself, and switched cords and stanchions.

Moved to another section of the marina and tried their stanchions.

No go.

It DOES run on the generator. So I assume it is either the marina itself (no other boats complaining of issues) or I have an issue at the shore power source.

Any ideas?

(1999 380 Sundancer)

Anyone?

transfer switch turned to generator and not shore power.
bad transfer switch
make sure ALL breakers are turned off
connect shore power cord, turn on tower main then turn on shore power main, then 1 breaker at a time to isolate the trouble

SOLVED! It took some sleuthing, and some help from a qualified electrician. It is my salon air conditioner. Not sure what component, probably the compressor, leaking just enough neutral to pop the GCFI - regardless of whether it was on or off at the breaker.

I completely disconnected the hots and the neutrals and taped them off and put everything back together. I can get by just fine on the forward air conditioner. A problem for the off-season.