So, a few questions and "what ifs" are in order:
1. Did this just start to happen? If so, has anything changed from when there was no flickering on gen (like new LED|’s installed that happen to flicker now when on gen only)?
2. Are all LED lights the same make and model.......or are the ones that flicker different than the ones that do not flicker?
3. When running the gen are other 120vac loads turned on like Air Conditioner(s). Try running only the gen feeding converter to see if anything changes with flickering.
4. Are your coach batteries fully charged or low on charge? Heavy or low loading on converter (depending on converter manufacturer and age of unit) may cause pulsing......but you said shore AC power is not a problem. So likely converter is OK unless it is allowing some electrical noise through from gen.
5. Both shore and gen feed the converter (120vac side) so flicker seems to be only an issue on gen.....test gen AC output voltage and frequency for stability. Might require an Onan AC output checkup if you don’t have the test equipment.
6. Check the transfer switch connections coming from the gen side are clean and tight (all shore 120VAC disconnected; gen off when doing this of course).
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