No disrespect, but the primary factor for establishing your battery capacity is the wattage required over time and available space. The second factor is maximum total load the battery bank is expected to support.
Your over time calculation has two subsets. Total wattage with no charging source, such as when dry camping. The other is loads in excess of what the charging source can supply. This would be when idling or idling at a rest stop.
Batteries have a limit on how much current they can supply at once. This will establish your minimum bank capacity.
My coach supports 2 roof airs and a host of other needs, like most other coaches with 2 Trace 4KW inverters. My setup uses 4 4D batteries, which is the smallest practical bank capacity for my coach's maximum load. We have dry camped using our AquaHot, no air conditioners, and typical house systems. Using a practice many if us former boaters use, we start the generator for about an hour or two in the morning to cook, make coffee, and top off the batteries. We do the same at dinnertime. With this routine, and our typical loads to include heating coach with our 3 zone Aquahot, our generator never was called upon. It's rare for us to dry camp. So, rarely would my batteries be subjected to this depth of discharge.
As for Featherlight's lack of deducated generator start battery, I'm not really a fan. They do have the option to start from either house or chassis batteries. Without adding a battery, I personally would rely on my chassis batteries to start the generator eliminating any risk the house bank was too depleted to start the generator. Yes, I know the generator doesn't require much power to start. I'm not suggesting a change, but adding a generator battery would be easy. It would maintain its charge through a Balmar Duocharge device that would both limit its charge from the house bank and allow for a different charge protocol, if needed.
Early on Featherlight coaches were the coaches of NASCAR. As such, they were built with the intention of supporting prolonged battery use.
In confident some conversions were equipped with a huge complement of batteries simply driven by marketing fears and stated metric of what that bank capacity would afford a buyer. Take my last coach, a 98 Country Coach conversion with a bank that was equipped to hold 8 8D batteries and typically sold with 6 8D batteries. This was a 12V coach with 2 2500 watt inverters and no possibility of powering air conditioning. This was a dry camper's dream coach. It spent 3.5 days on a ferry with nothing running but the refrigerator. After that time the bank was still at 70% of capacity. While CC was installing 6 big batteries, Liberty's 12V coach had 3 batteries hidden out of sight.
I have participated in the bank size reduction on several coaches with not a single owner disappointed in their decision.
Some owners like to spend a lot if time off the grid and don't want the noise of a generator. These are great candidates for lithium battery banks and solar.
The bottom line is your use should dictate your battery bank's capacity. If you don't the potential excess weight of an oversized bank or $1,200 per 24V battery equivalent, fill the manufacturer"s battery storage area.
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