I use a home made current tester to check for battery drain and other amp drainage issues. Course a multimeter would work too but I prefer the following method.
Solder a couple leads onto a std 1156 automotive bulb. One on the case and the other on the large solder connection in the moddle of the bottom of the bulb. Use at least 10" of wire per connection and solder/screw on an alligator clip on the end of each wire. To test the tester, place one wire on the battery + and the other on battery - and the lamp should light.
Now, disconnect the battery + terminal connector from the battery and connect one of the testers leads to it and hang it to where it doesn't touch anything else. Connect the other tester lead to the battery's + terminal. (you can do the same with a multimeter setup to read amps). Basically you have connected the tester in series with the battery and load (tractor).
With the key off the lamp should not glow. Not even barely. If the lamps glows you have a current drain large enough to drain your battery. (The charging circuit will have a very minimal drain but this is not enough to light the lamp).
To determine what device is draining the battery, disconnect one fuse at a time until the lamp goes out. If find it this way trouble shoot the effected circuit accordingly. If this doesn't isolate it, you will then need to check the non fused devices by disconnecting the "hot" wire from them one at a time. These are the starter and charging circuit and/or any circuits you may have added that connect directly to the battery.
I've found bad regulators, oil sender units and a shorted hot wire in this manner.
By the freq of you battery discharging, this may not take you long to find.
Good luck