SteveH's recommendation is the right place to start, check that all of the connections are secure and clean between the batteries and the fuse block.
If you find that the connections are all good, the next place to look is the state of the batteries and the converter. If your batteries are original, or even from 2010 or 2011 they may not be in good shape (especially if the trailer has been plugged in a lot with the original converter.) Also, the converter may be having issues. Measure the converter's output voltage and if you can, take the batteries to a shop that has a battery load/charge tester (auto parts stores often have these.) If you're not familiar with the battery removal process, be sure to unplug the trailer and to isolate the battery cables for the first battery while disconnecting the 2nd (assuming you have 2).
There are several possible causes of this symptom. For example, I replaced a converter that charged the batteries well enough, but when it should have been in "float" mode it would vary the output voltage every minute or so and cause noticeable flickering in the lights that annoyed me.
Il Carriaggio — 1975 Argosy 24 | Il Progetto — 1976 Argosy 28 Center Bath | WBCCI# 15566
He has all of the virtues I dislike and none of the vices I admire. — Sir Winston Churchill