If you have 2 house batteries you need to replace both of them as you can't have 1 good one and 1 bad one working together. It's possible you only have 1 - 12V for the house, I happen to use 2 - 6v, in which case you would only need one.
As far as the generator goes we bought ours in 2020 from the original owners, so it was 14 years old, and the genny only had 22 hours on it. I was a bit concerned but after listening to some other owners in the Roadtrek groups I purchased new batteries, filled the gas tank with gas and added 3 cans of Seafoam to it. It started and I just let it run for a few hours. It was good for a couple years then I had to replace the fuel pump and have some other work done to it. So if it's me I'm trying the Seafoam first just to see what happens. I'm 71 now and there's no way I'm crawling under that thing to drop the genny


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Please don't rely on AI for your answers just join a few Roadtrek Groups on Facebook, or other platforms, as the folks there are really knowledgeable and more than happy to help with any questions you might have.
Good luck!