3 Group 27s with some wiggle room? Or 4 GC2 Sardines?

Sounds like more than I’d care to deal with.
If you really want a bunch of reserve capacity, and plan on using it, might be money well spent to go lithium.

Sealed lead acid is an option. No need to check water. There are 200ah 6v that are shorter than Trojan batteries. If you stacked them you wouldn’t need to touch them again.
Mighty max batteries from Amazon. Worked well for me in space limitations.