My experience indicates you have a bad glow plug controller and / or sensor.
Depending on the machine model, may be one or two "black boxes" under the dashboard (on mine, right of the steering wheel). Get a parts schematic and see where yours are and where it / they are mounted and how wired.
The problem is that the controller is keeping the glow plugs "on" continuously when the key switch is also "on"---the glow plugs are using so much electrical current from the battery that the tractor charging unit (alternator, generator, dyno-whatever) can't keep up, so the battery goes dead.
Pull the dash and go for the control box. If two boxes get both at dealer and make a deal to return the one not used. Mine was ~$70 couple years ago.
Don't plug it in----use jumpers so the connectors stay nice and unmarked. Then based on the deal with the dealer, return the unused part.
Switch out one box then the other to pin down where the problem is---don't install both new boxes at once or you can't determine which part is bad.
Don't worry about the glow plugs, they are probably fine.
I bought a tractor from a guy had the same problem: what he did was put a cutoff switch on the battery. On that machine he turned the battery master switch 'on' to crank, then 'off' to operate the machine. And he'd stick a charger on the battery for a day every month or so. Worked for him.
If the battery has been repeatedly discharged fully it may be mostly ruined. You can try using a battery tender-maintainer to see if it will give a bit more life to a weak battery. $30 or less on sale at Tractor Supply. Avoid Harbor Freight battery maintainers as will boil battery dry.
Please post back your continuing experiences so we may all learn.
