Life span of a battery

I do think the key to battery longevity is using the vehicle it's attached to, I don't use my landy as much as I'd like, it can sit outside the house for weeks sometimes. I do disconnect it when it's not being used but in the 6 years of ownership it's had 3 batteries. If I use it every day it starts no problem, if I don't, it struggles to start. First I thought it was a starter problem so I replaced that, then I thought battery so I replaced that, then I thought dodgy earths so I replaced the cables and cleaned everything. Now I just think it's because I'm not using it enough.

Col

When not in use could you take batt off and keep it in doors ? And then trickle charge say once a month?