[C320-list] 120 volt power circuit issue

[Dave Hupe]

I have 2 ground fault circuit interrupter (GFCI) outlets on the 120 volt AC power side that control all of the AC outlets on my 1994 boat.  I've had them trip a couple times lately, but only when I come back in and reconnect shore power.  I hadn't been turning off the master breaker switch at the stern (as I really should) before I connect or disconnect the shore power cord.  When I did turn the switch off before disconnecting/reconnecting, the GFCIs don't trip.

I thought that maybe the GFCIs were "tired" (probably original equipment) and/or I'm getting a small "zap" when I make the shore power connection that is making them trip. I figured it might be a good idea to install new GFCIs (assuming old and possibly the problem).  So, I bought new 15 amp ones.  I installed one, turned the power back on, and it tripped immediately each time I applied power.  I reinstalled the old one and it did not trip.  I returned the 15 amp units to the store and got 20 amp GFCIs (I finally saw on the old ones that they were 20 amp).  However the same thing happened .... It tripped, but the old one doesn't when reinstalled.  

The new GFCIs reportedly do automatic  "self tests" every 3 seconds per the packaging.  Other slightly cheaper ones reportedly do tests every 1.5 minutes. So, I don't know if these new GFCIs are more sensitive than the old ones (which likely don't self test), or is the self test feature itself a problem?

A friend with a newer 320 (year 2000) mentioned about having an isolation transformer on his boat and wondering if that would interfere with the new type GFCIs.  I think we've resolved that he actually has a galvanic isolator that Catalina started installing in 1998 https://www.catalinadirect.com/shop-by-boat/catalina-380390/electrical/galvanic-isolator-30-amp/  I'm not sure if an isolation transformer is really different than a galvanic isolator.  However, my boat has neither being older. 

I'd like to resolve this to feel safer, but don't know what to do.  I wonder if I have a poor ground connection related to the AC outlets? 

Any ideas/similar experience?
Dave Hupe
1994 Cat 320 (#32) "Mayan Sun"
Holland, MI