Many of the computers and electronic gadgets in modern farm equipment draw a little power even when the key switch is off.
As long as they're connected to a battery, they draw a few milliamps of current to power built-in memories, clocks and other functions. A few milliamps for one computer, a few milliamps for a GPS system, a few milliamps to feed a memory in a monitor display, and before long you've got enough of a "draw" to drain a machine's battery if it's stored for more than a few weeks.
The owner's manual for a brand-new combine recommends that owners switch off the machine's built-in battery disconnect switch if the combine will be left idle for more than two weeks. Interestingly enough, the manual recommends completely disconnecting the negative leads on the battery if the machine will be stored for more than 90 days.
Check your owner's manual(s) before disconnecting all electrical power. On cars and pickup trucks, disconnecting batteries can wipe the memories of radio station presets and other convenience functions. Farm equipment may have similar losses of reprogrammable information, but totally removing battery power shouldn't harm any electrical components and will improve battery longevity.
Be sure to fully charge batteries before disconnecting them, and occasionally trickle charge them if storage lasts longer than three months.