I always look at replacement between the 3rd and 4th year. Why? Because I've seen degradation beginning then, particularly faster discharging. This is my guideline for a properly maintained battery. Poorly maintained batteries can be lost a lot sooner.
My Marlin came with Deka batteries, GP27.
In searching for replacements I came across Interstate SRM-27B Deep Cycle/Cranking. They have the GP27 physical size but pack more punch and reserve. Don't confuse with the SRM-27 - not the same batteries. The B series is generally not carried by distributors and must be spacial ordered. They are horses and will run your electronics for a long time with plenty of startup power for the run home.