I have a 2004 T190 that I suspect has a leaking case drain filter. The symptoms sound familiar.
I didnt realize it was leaking until I put everything back together.
While servicing the drive motors, I found several inches of muck inside the tub which I cleaned out and one of the aluminum filter...
I would say to definitely get the block heater and use it. My T190 is only used occasionally during the winter, mostly to plow snow.
I plug in a few hours before starting. It makes starting much easier on the engine and the battery.