Congratulations on you new to you purchase! I am excited for you!
I lived in Marquette so I know the snow you are in for! I hope to get back there soon.
For my trailer before winter I always:
Check on the roof for any areas needing any marine caulk,
Check the roof to make sure vents etc. are secure,
Check around the windows
for gaps in rubber/caulk,
Check all the bolts on the frame to make sure they are tight,
Steel wool any surface rust on the frame,
Spray the frame with rust preventer and a rust protector.
I never winterize mine so other's can better help you out with that.
Check out the Fiberglass groups on the RVillage app.
Photos, Caravan, Boondocking, Make New Friends!