What kind of converter do you have? Most converters have active filtering built into them, but there are a few older ones that don't - they convert the incoming AC to pulsating DC and feed it directly out without any kind of filtering.
If you have one without filtering (what kind of a converter do you have?) your LEDs will pulsate at 120 Hz when you're plugged into clean shore power. This is too fast for the human eye to see, so everything looks fine.
Few RV or portable generators put out power as clean as what you get from the local utility, so if your converter does not do any additional filtering (again, what kind do you have?) you'll get additional flicker as the DC voltage follows the generator's pulsing power.
If your generator's voltage varies outside it's spec, the answer is to fix the generator. But if it's within spec, and you don't have an electronically filtered converter, the easiest solution is to upgrade your converter to a more modern unit that has an output that remains constant as the incoming AC voltage changes.