OK, now that I'm awake, I'll try to be more lucid.
Although it is not exactly what you proposed, the simplest and best way
to provide an additional 24 amps for the oven, fridge, etc., is to add a
second 30A cable, breaker, isolation transformer, and associated wiring.
The isolation transformer is especially appropriate for a steel vessel.
The cost would be greater if you were to install a 50A isolation
transformer and the benefits would be marginal in my opinion. Having two
independent branch circuits offers some redundancy and doesn't require
that you mess with your existing AC wiring.
Connecting the two 30A cables through a Y to a 50A shore power
receptacle would be OK since it is extremely unlikely that all of your
electrical loads would ever be on at the same time. Even if they were,
presumably the combined load would not exceed 54 amps, which, under a
worst case, would trip the shore power 50A breaker. Virtually all
residential branch circuits in the US are sized based on the assumption
that all loads will not be present simultaneously. Of course, if you
know the assumption is false for your boat, that would require a
different approach.
While you could combine the secondaries of the two isolation
transformers and have a single large circuit, I would recommend that you
keep the new wiring separate from the old. Needless to say, combining
the output of the existing isolation transformer with a new, un-isolated
30A circuit would negate the benefits of the existing transformer.
Richard's point about using a 30A cable with a 50A receptacle is
troubling. If that is what you are doing, there is a real risk that the
30A cable could short, cause a fire, etc., without the 50A shore power
breaker tripping! The Y connection would be much safer.
Apologies for the false start.
Chuck
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