I don't have a lot of experience, but our batteries survived an unusually cold Colorado winter last year without any trouble.
I filled them, charged them, and then visited every month or so and ran the engine (not the generator) for about an hour at 1500 RPM. This keeps both the chassis and house batteries charged quite nicely. According to Motorhome magazine it's quite a bit more efficient than running the generator.
Of course, plugging in a real charger would work well too. I wouldn't count on the coach's converter/charger being up to the task. There are known issues with it. A good, 3 or 4 stage, external charger would be great though. Or, replace the converter with a good after-market model.
I have installed a cutoff switch on the chassis battery. There is a significant parasitic drain on it that I was unable to find. I throw the knife-switch whenever I put the coach in storage solving that problem.