To answer the original question about how waterproof tractors are... they are pretty good if you cover the obvious places that water can get in - the seat, the dash, key slots, switches, any shift levers or such on the floor that has a rubber collar or bellows, the center of the steering wheel!, and all the filler caps and dipsticks.
Since those are rather too much to deal with, what it comes down to is to get a good lightweight cotton canvas tarp - because it breathes - and tarp it when it's parked for the night. The tractor will be fine. Otherwise they age real fast.