More info is needed ... are you the original owner of this 2007 trailer, if not perhaps some of the bulbs have been changed to LEDs. If they're all 921 incandescent bulbs then perhaps the converter is the cause, in which case turning it off and running on battery power might solve the issue. If that works then it's possible the converter is failing or there's a loose ground ... but that could be just about anywhere and very difficult to locate, though checking the connections at the converter would be the obvious place to start.
I'm not the original owner. I traded trailers last fall. Upon checking it out there was a problem when the generator was running so ultimately they installed a new converter.