Do the bearings now. If they're fine then you won't have to do them in spring. If they aren't you've got the winter to sort them instead of a nasty surprise when you want to start using the boat again.
If you have a braked trailer then it's worth taking the cables off & oiling them in the same way as bike/motorbike cables - I hang mine up & put a small amount of oil into the metal cup at the end of the cable (if you haven't got that type than a cone of gaffer tape or plasticine works). The oil works its way down the cable & any excess drips out into a tray you've placed under the end. Takes time & I usually leave them for at least a week. Never had one seize.
Either take wheels off & store in dark place or support trailer on axle stands to take weight off tyres.
Inspect your tyres. Any cracking then replace as you don't want to be stuck at the side of the road!. Tyres deteriorate over time so check the date of manufacture & if they're old then replace. It's a common cause of tyre failure on caravans - the tyres will have little wear & look fine but they aren't.
Worth a read: Tyre Ageing
Tow hitch. Check & grease as required .
NB. If you have the type of hitch which has brakepads inside the hitch which act on the towball then don't grease or lubricate either the ball or the inside of the hitch.