I'm in MN as well, here is what I have done for the past 6 years without issue in the fall for shutting down the boat... No sense paying $250 for that service if you ask me. After buying a oil extractor and a couple other tools the first time, it's ~$50 for oil and filters for 2 engines each year and $30 for some other supplies I have below for actual storage. Feel free to let me know if you have any more questions.
- Run Marine Sta-bil thru the gas starting in Sept. Fill the tank one last time before changing the oil adding a bit more stabilizer to up the ratio a bit for storage at that time
- Change oil and oil filters after last time out.
- Apply a bit of fogging oil thru the spark plug holes after shut down, turn over engine for a couple seconds (no hose) with lanyard removed so it doesn't completely start.
- Blow out hose lines with air compressor from where you attach the hose just to get rid of a small bit of water.
- Depending on where your storage is (indoor, outdoor, pests, no pests, etc) here's a couple other things to consider:
-> Tape off a couple of the hull fittings (bilge, cooling pissers, anchor locker, etc) just to prevent things from getting in.
-> When heading to storage I put dryer sheets in various places and also shave Irish Spring soap to prevent mice, not sure if this works, but I haven't had any mice issues.

-> I also grab a couple Damp-Rid containers and put them in a couple low spots, like between the engines, ski locker, and in the helm compartment.
-> I know people that put their trailer on jack stands to get load off the tires (I haven't done this).
I have never ran RV antifreeze, never had any issues and you know how cold it gets up here. My storage is indoors, not heated or insulated.