If you are planning to go away for a few weeks - I wouldn't worry about it going off. Do not leave water in there unless you need it.
Leaving the water in there will only add to the weight of the 4x4 and chew the fuel. If you can fill it in a caravan park while you are away.
Are you planning a huge trip where there is no water at all?
If you carried 3 10ltr water boxes that would be more than enough for a week.
We have a 70 litre tank in our Camper Trailer and never use it unless we go somewhere the water is suss or scarce. That has only been once.
Aussie Disposals sell tank tablets - really cheap too!
Any man, in the right situation, is capable of murder. But not any man is capable of being a good camper. So, murder and camping are not as similar as you might think.