Find a very small 6 volt bulb, like from a 6 volt flashlight that uses the single square battery. Or borrow one from the dash on the tractor. Unhook the battery cable, either positive or negative, doesn稚 matter.
Rig up some leads on the bulb so you can hook it between the battery post and the disconnected battery clamp.
If you have a drain, the small bulb should glow.
Go around and unhook wires, pull fuses, unplug connectors, one at a time and watch the bulb. If it stops glowing, you have found the drain. Reconnect each wire or circuit before unplugging the next one.