To the OP......You need to study until you get a grip on it. 6v batteries have their place in a house battery application. But they are remnants of days gone by. Depending on how many amp hours you want or if you want to start small go larger, you can minimize your total investment by going 12v. 6v in series Vs 12v in parallel. There are pluses and minuses to each one. There comes a point where 6v are more practical than 12v. but in a 300 to 450 amp hour system you cant beat the weight per dollar per amp hour of the 12v system. In the larger systems 6v still make sense.
Its about amp hours used. You charger has to put back what you take out. It goes back harder than it comes out. So efficiencies suffer. What is happening is your generator is running a charger that bulk charges, this is a rapid charge but can only bring your batteries to about 75 to 80 % of their capacity. The rest of the charge is accomplished at an ever decreasing charge rate of about 3 amps. It is this acceptance charge that is running your generator. Acceptance runs about 6 hours. So 1 or 2 hours for bulk, and 6 for acceptance. Total 8 hours for full charge.
1 amp hour out you got get one back in, they only go back in at about 80% of what they came out at.
This is a typical problem for people that have house batteries that are trying to charge with a generator. Cummins Onan has a chart that will tell you how long it takes to charge a 50% depleted house battery. This is where you answers lay.
Do a lot of research before you go out and buy into the 6v costco golf cart battery solution, You will be spending money on what you think is a solution but more money down the drain.
While there is debate on 12v vs 6v, there is an analysis that you must do before you decide. The house battery recharge and usage is larger than a couple of costco batts. I promise you.
I will find the Cummins chart for you and provide the link, but just think if you had a small battery bank that was 50% depleted, you would have to drive 600 miles to charge them and you still would not have them charged up. I will get you a few links so you know what to start looking for. The house batteries that you need cannot be bought at Sears, Costco, Napa, WalMart, or AutoZone. Not even interstate battery has them. You need industrial batteries that are designed to be charged and discharged to 50% to 100% on a daily basis, batteries that have an owners manual, that tell you how long and at what voltage and amps to use. Study before you spend another dime. I will also put together some math for you to look at so that you can see what you are up against.