daed batteries
There are only 3 possible reasons they are going dead.
1) There is some load (drain) on them
2) The batteries are defective
3) Someone is using them without your knowledge
To check 1) put a current meter inline to all wires leaving the battery (or returning). There are some things which are normal (like the Propane Detector, which will eventually drain any battery), but a substantual current is likely your problem. Finding what is sucking the current will be a chore, unless whatever it is has a light or makes a noise.
To check 2) fill the water to the recommended level (if applicable) and then charge them up fully (recheck the water level), then completely disconnect them from all wires. Leave them this way for the few weeks, then check them. If they are mostly charged (all batteries 'self discharge' some), then apply a heavy load (a battery/load tester or something which uses a lot of current) and ensure it passes/powers the device for a reasonable period of time. If the batteries pass both these tests, there is nothing wrong with them.
As for 3, that is a security issue. Locking access, hidden cameras, light dusting of talcum powder, etc may provide some insight.
Note that discharging a battery more than 50% can lead to a reduced life.