I would suspect the converter is not able to keep up with the load when the charger kicks on.
What is the voltage when the lights are bright and what is the voltage when the lights are dim?
Ahh crap I didn't mean to do that, I can't check the voltage when it dimes because it only dims for a fraction of a second each time but the non dim voltage is 12.6
if your voltage when the lights are bright is 12.6 it means your converter isn't working at all if you are plugged in. what is starting that causes it to dim, the battery can only handle so much especial when it is down to 12.6v already. you threw me off as in your original post you said the converter is giving the proper voltage but 12.6 when the converter is on isn't proper, not even close.
I was told 12.6 was what the output should be when plugged in I take it to mean that's not the case. ok, what should the output voltage actually be? And would this mean there's a problem with the converter or the batteries?
pluggted in your converter supplies the power, and depending weather it is in bulk, adsorb or float. Also depends what battery type it is set for and what brand/model the converter is. at any rate it will be over 13V when plugged in, when not plugged in you will be running off the battery power so the highest you will see is 12.8V on a fully charged flooded battery, LifePO4 batteries will be higher.
Just checked again, is 13.6 more accurate?
Yes 13.6 is definitely a sign that the converter is working.
If the dimming only occurs when you're on shore power the voltage is probably briefly dropping to 12.6 or battery voltage. It could be the converter is starting to age. Or it could be a loose or corroded ground. A loose connection would allow the meter to read 13.6 volts. But it might not be able to keep up with a sudden demand for power.
Since not everybody (myself included) have a plethora of fancy testing equipment I always try to think of different ways to test a theory. If you're dimming is kind of predictable then I would try something like connecting a standalone battery charger to the batteries. Then with a second source of higher voltage in the system see if the dimming occurs less frequently or even stops.
When full charged (and the charging stops), the battery should be at 12.7v (+-0.1). When charging, it should be somewhere on the order of 13.2-14.6 (modern chargers have different charging stages with different voltages).
What type of lights do you have? (LED or Incandesant)
