You can use a deck mount converter as a portable charger. It does not replace the existing fuse panel etc. Just have a set of cables from the converter's output to the battery posts. I use a set of cut off jumper cables (at least #8 or fatter) with the clamps.
AGM battery posts are hard to clamp to so you might choose ring terminal lugs on the wires.
When on shore power, you can plug in the converter to any receptacle near enough to the converter attached to the battery. You can leave it there like that and set it to the Float voltage you want. That will depend on the temperature where the battery is but around 13.6 is normal for 25C
Here is how to operate the LK with a little screwdriver in the hole(s) and a flashlight so you can see in them! :)
https://powermaxconverters.com/wp-content/uploads/2021/04/PM3-Owners-Manual.pdf