DO I need REALLY GOOD house batteries if i NEVER boondock ?

Yes that will work.

CA Traveler wrote:
The battery only needs to be good enough to start the gen.

Yes one battery is enough and it doesn’t even need to be very good .. use the boost switch to help start the generator.

We ran our generator for 7 days while the area was recovering from a windstorm a few years ago. Our "house" battery was a single Walmart MAXX Marine Group 29 maintenance free. Worked fine...

Two thoughts. First no, if you always have shore power you don't need a kilobuck of batteries. If you can get at 'em and feed 'em a bit of distilled a pair of standard lead acid flooded wet GC-2's is more than enough
or a group 31 (in Maintenance free or AGM)

However second thought
Digger O'Grady and his marvelous backhoe did a lovely job on one of the high voltage primary lines into the campground where I was camped.. I spend a couple days on generator/battery

Storms did the same at another CG

Three More had underground lines fail.. One while I was absent, that one was "Selected sites" so all you needed to move (About half a dozen sites including 2 of my favorites) another it was the main feeder to ALL the park, and another to about 1/3 the park. That all park one I did not have a generator (PUP days)

Heck, underground feeder took out power to the next building in my apartment complex a week ago tomorrow. 16 and a half hours

I have battery backup for SOME stuff here. .Not sure I could power the fridge for 16hours though.

One factor to think about is whether or not he ever wants to run the furnace without shorepower or running the generator.

While I don't boondock much, I've been in a few situations where I wanted some heat but didn't want to run the generator, I've got around 5 or 6 hours of safe furnace fan runtime in the batteries (continuous runtime), which should last at least for a typical night of intermittent generator use.

Though unless he has a high powered furnace fan, that wouldn't need $500 worth of batteries.

If he never needs to run the furnace, then a very small battery would be enough to run the lights when temporarily parked would be fine.

"Is my logic sound?"

Yes.

BTW A Marine deep cycle battery is a compromised starting battery and not a deep cycle battery inspite of the label.

The battery only needs to be good enough to start the gen. He could use a flooded battery with it's maintenance and shorter life normally but I'd recommend a AGM either 12V or 2x 6V which is more expensive but no maintenance.

U R correct.

If you don't boondock, we never do, is to have the charge in the house batteries to start the generator. Without the batteries being good enough the Onan will not start and stay running. Happened to me two years ago.