I have found room for the 5th battery.
I'm really surprised though that it pulls down the new AGM.
I pulled the Optima off my boat at the lake and took it to the shop and had it tested. It was full charge and strong. It is a mystery to me but I hope the 5th battery does the trick.
on mine if i fish a small lake with df and live wells on full time all day and very little driving around to charge it up mine will drain down to where my outboard wont crank the tell tale is when my df beep low voltage and drops below 10.2 v and shuts off. thankfully i have a jump start switch that paralells one or all the trolling batts to the starter battery and I can start it up. I had an accessory battery in the bullet and that cured the problem