If this is going to be a new to you boat/trailer, you should bring two sets of bearings, grease and all the tools to change the bearings. Bring, at minimum, a spare tire (preferably the whole spare wheel.) I also agree to have the boat shrink wrapped if it can be done.
Also make sure your tow rig maintenance is not ignored. Tires, fuel, cooling system up to snuff. If it starts to snow a little on you, fill the tank up, even if you have half a tank. If you have never towed in the snow before, give yourself enough distance to let the truck coast to a stop. That means go slow enough so that you can coast to a stop and give enough distance to allow the coasting to occur. Over cautious will keep you out of the ditch.
How many miles will be in potential snow? To/from where? If you're going from Maine to Idaho, that is like a 2 week trip in the snow with a trailer. I suspect your trip is not as I mentioned, but some people may be able to help with local winter conditions if you want to be a little more specific.
Not to scare you, but I'd be nervous as heck about this. The good thing is that with enough planning, it will not be too bad. I also think it would be worth rerouting the trip to avoid any storms.