One thing i highly recommend in addition to the standard winterize of the motor is getting the boat shrinkwrapped. It will help keep moisture at bay and you wont have any issued with snow buildup etc. Its truly the way to go vs covering it with a tarp.
I also put some mothballs in a cup at the vent point where critters may get in after its been wrapped to keep them out.
Other things to do, get a set of those cheap RV tire covers and cover the tires so they dont get beat to death by the sun. Spray any metal on the trailer that can rust with a corrosion inhibiter, I also remove all tension from the trailering straps and if you can jack the trailer and put it on blocks so you dont risk flatspotting the tires.