We kept our boat in the water, and used it, throughout last winter. It was an unusually cold winter for the Puget Sound. During the coldest spells we ran a small ceramic heater. We propped up the step/cover in the cabin over the engine compartment, as well as the door to the head. The R21 will be somewhat different.
Most of the winter we just ran an Air Dryer/De-humidifier
. And, we used the boat at least twice a month. No problems.
We keep Moondance in the water, so even when we get the occasional sub-freezing temperatures, she is sitting in 50+ degree water. I don't recall if you trailer Red Ranger. That would make a difference.
Glad to know you'll be out and about between now and spring. Hope we cross wakes.