Edit: TL;DR: You may want to see if you can adjust you refridge's low voltage setting and/or clean up wiring before adding a conditioner to the voltage and the conditioner you are thinking of is likely a device known as a "DC Boost converter", a "DC Step Up Converter", a "DC Boost Buck Converter" or an "Adjustable DC Converter". A transformer is for AC, not DC.
Transformers are usually for AC current. 12v on boats is usually DC current. I think the device you are thinking of is called a "DC Boost Converter", "DC Boost Buck Converter" or "DC Step Up Converter".
Transformers use electromagnetic coupling, i.e. induction, across two coils to adjust the voltage. Transformers scale one voltage to another voltage by a fixed ratio, i.e. the number of turns in one of the coil to the number of turns in the other coil. But, they do not stabilize it. If there is fluctuation at the input end of a transformer, there will be the same fluctuation, as a percentage, on the output side of a transformer. And, because transformers rely on flux, a property of AC current, they won't work for DC current. Commonly available units also tend to make big changes, e.g. 2:1 or 1:2, not 1:1.108.
A DC boost converter is a form of switching power supply that can both increase and stabilize a DC voltage. A DC buck converter lowers and stabilizes DC voltages. A "DC Boost Buck Converter" can take a range of DC inputs, a little above to a little below the desired voltage, and produce a reasonably steady DC output, e.g. 9-18vdc input is rendered to 12vdc +/1 0.1vdc output. Boost-Buck converters basically work by internally switching on-and-off really quickly to approximate an AC signal and the feeding that to an inductor that produces output at about the right output voltage, and then conditioning that output further, often times with capacitors to smooth out ripple.
I've used boost-buck converters to stabilize voltage for very sensitive DC devices -- but never on a boat. On boats I've used boost converters to run 24v devices from 12v boats, and buck converters to do the opposite.
In theory, one of these devices could be used to resolve the problem you've got. And, there are plenty for sale on Amazon and elsewhere, many fully waterproof. They tend to be somewhat inexpensive, even for reasonable wattages.
...Feel free to search amazon or elsewhere for a "12v dc boost converter" or "12v dc step up converter" or "adjustable DC step-up converter" of an appropriate wattage and input range.
But, I wouldn't jump to one as a solution. It might only be a band aid for a problem that could make itself worse in other ways. Or, even easier it' may be a fix for a non-problem: The refrige may just needs a setting change.
What is the cause of the voltage fluctuation? How much is the voltage fluctuation? And, what specifically is your goal with respect to how low you want to go?
If you are trying to squeeze out a little more runtime from the fridge as the voltage from the batteries slowly gets lower from use, it is probably unnecessary to use a converter. Most popular refrigerators allow you to change this voltage cut-off, either via soft-settings or by changing a physical setting on the converter. My refrigerator and freezer both use a controller that let's one change this setting changing the value of a cheap common resistor:
--
https://www.secop.com/fileadmin/use...c_unit_ac-dc_101n0510_08-2019_desi101g202.pdf
If you've got a lot of resistance going from your batteries up to your devices such that, even when your batteries are fully charged or your alternator or charger are putting out good voltage, it is low by the time it gets to the device, you probably want to clean up all of the connections in-between so that they aren't high resistance.
If the voltage suddenly drops when some device is active, you probably want to check to see if that device is working normally, if there is a connectivity problem, and/or if your batteries and supplies are operating normally.
I wouldn't use a boost-buck converter instead of adjusting a setting on the refrige.
And, I wouldn't use one instead of cleaning up a wiring problem.
But, I guess I might use one to fix a really sensitive device that wouldn't allow for calibration and was otherwise operating normally.