As a followup to my original post, I have not yet tackled removal of the 110 volt to 230 volt transformer to see if it can be repaired, nor have I been able to find any blown fuse or tripped circuit breaker on this circuit. This has been a low priority because instead I have been able to find 220 service throughout the Caribbean, and use the 220 cable instead with the appropriate US or EU adaptor.
Nonetheless, when I installed a metered galvanic isolator like the one BeBe (Bill Rouse) posted in the photos section, I had the electrician install it next to the 110/220 volt plug box on the engine room forward bulkhead, connected in series to the ground outgoing cables in such a way that it would operate no matter whether the 110 volt cable or the 220
volt cable was in use. Thus, no matter which cable is connected to shore ground, the galvanic isolator is connected and functioning to prevent stray current via the shore ground.
I realize that this response does not directly answer your question about whether the 110 to 220 volt transformer acts as an isolation transformer. That question is beyond my expertise. However, I do know that the ground wires coming into the 110/220 volt plug box on the engine room forward bulkhead go to a common point within that box, and I therefore expect that stray current/voltage from either of the shore cables would enter the boat undeterred by the presence of the 110 to 220 volt transformer (and hence the need for the installation of the galvanic isolator).
I hope others with more electrical knowledge than me will address your question better.
"Currently" in Sapphire Beach Marina, Saint Thomas, USVI