So it starts from your two 6 volt AGM batteries? Have you checked all the battery cables and connections? It sounds like it may not be getting enough cranking voltage sometimes which could be an intermittent connection.
I would expect the 12v lights in the RV to dim when the generator was cranking. This should only last for a few seconds while it starts but would be perfectly normal. How long do the lights stay dim?
I'd find out how much a starter is for the generator and how hard it is to change. If it's not too bad you might get one for a spare and keep it with you. Or, have them change the starter and keep the old one as a spare.
