Hey Kthoennes,
Glad your refridge project worked out! Congrats!
One can certainly do, as you did, and use an inverter with a transfer switch point-of-use. And, it certainly can be perfectly fine and safe.
But, we now have shore power and battery power in the same box, input and output AC lines that need to be connected and disconnected, and a G-N bond that needs to be made and broken. And, have to make sure that the chassis has proper grounding so it can carry DC current back to the battery over thick battery wires not thinner AC wires. What concerns me about these things is that the inverter can seem to work, but generate a safety problem, even if they are wrong.
I might be able to think up other potential wrinkles, too.
So, in my own personal case, I feel really comfortable suggesting that point-of-use inverters with no transfer switch are just DC loads and can be treated like other DC loads.
But, once there is shore AC and boat DC in the same box and/or a transfer switch involved I, personally, can't think of it as a load any more. I think of it as part of the source and distribution system. And that means I start asking the other questions I was asking, and in my own personal case, buy a listed unit where these things have been comprehensive asked, answered, and tested in a systematic and documented way.
In a dry environment, point of use in the cabin, I can see one using a UL 458 unit without the marine supplement. It should still get the bonding and grounding right, etc. Butbtjosebhwlere who call that a slippery slope are probably very right.