How do you maintain our little 12v camping equipment gadgets?

I have a 12v lantern with inbuilt battery, Coleman hot water system, etc.

From my experience leaving them for long periods of time not charging kills their batteries which I have found with my Coleman hot water system.

Lately I've been leaving my lantern on charge but forget about it and go down to the shed to find it's still in charge for the last month.

Dangerous?

Fully charging 12v gadgets, leave them for X months before topping up before they drain too long for X weeks/months, what is the best practise to keep them in order for the next camping trip?