RobWNY ... I recommend going with four(4) batteries if you have the budget and room in your RV.
The reasons for this are subtle, such as:
- If want to RV with a Boy Scout attitude then you need to be ready for a variety of situations, which in general includes having a large as possible amount of energy storage.
- The more batteries you have the longer they last with light average usage when camped, because batteries last longest when discharged the least amount during each usage cycle.
- You could wind up camped with hookups in a campground that experiences an extended power outage.
- You could wind up camped with hookups in a campground during an extended lightening storm in which you wish to disconnect from shore power to prevent lightening discharge travel through the cabling into your RV.
- You could wind up camping without hookups but being stranded an extended length of time due to equipment failure or bad weather.
- With four 6 volt batteries and one of them ever experiences catostrophic failure, you can then still have 12V power on board via two of the three remaining good batteries.
- You could wind up stranded for an extended time in a massive highway traffic jam due to unexpected weather, earthquake, flood, bridge failure, forest fire, etc.. such that you need to hunker down in the RV along the road.
- If you're able to store your RV at the location of your personal residence, then you may want to live in it for several hours/days without hookup power while remodeling work is being done on the residence requiring shutting off of the power at the property.
Having solar on board and a generator on board can also be justified as additional backup in situations such as the above.