Maybe I can help here by explaining a few things and letting you know how my 1996 Tollycraft's 12V ceiling mounted lights were powered. I chose "ceiling mounted" carefully as all of my wall mounted lights are only 120V AC powered. Perhaps yours are the same.
A transformer typically will only alter the AC voltage, nothing else. Transformers can not be used with DC. When a transformer is feed AC power, AC power at a different voltage is the result, isolation and polarization transformers are an exception but are not part of this discussion.
When my boat was built Tollycraft powered all of the ceiling mounted incandescent Xenon lights with 12V DC (from the batteries) as they should be, but they also installed Lighting Transformers which when turned on shut off the 12V DC and feed the ceiling mounted lights with 12V AC, and they worked. Any incandescent bulb doesn't really care about the wave form, they are in effect AC/DC, they work with both although with AC, although they might be dimmer as the power will be 12V AC from the transformer rather than the 13.4V DC from your batteries and if your eyes are quick, they might slightly flicker.
Along with the Lighting Transformers there will be an equal amount of relays (likely the size of a large pill bottle) that when you turn on the Lighting Transformer's switch or breaker will switch the power feed to the lights from 12V DC to 12V AC, sourced from the 120V AC powered transformer using the same wire, as it's also AC/DC.
The reason for all of this electrical hardware is that it removes the lighting load, which was considerable prior to the development of effective task lighting LEDS, from your batteries (and your charger) while you were plugged into shore power. Battery charging at least to float, can then happen quicker.
Switching out the bulbs in your ceiling mounted fixtures to LEDS renders the Lighting Transformers useless in powering your LEDS unless you also have rectification, which is doubtful. LEDS don't run on AC, they need DC.
LEDS also eliminates the need for Lighting Transformers, their relays and the relay's coil rectifiers as the now lighting load is so small compared to incandescents that we can live with the battery or charger powering all of the lighting, all of the time.
Your boat may be configured differently, I've never owned a Mainship.
My boat was configured this way.
I switched to LEDS years ago. The Lighting Transformers, relays and rectifiers are presently in ships stores, waiting 10 years now to be repurposed.
Hope this helps.