Just wondering if it would be wise to put a ziplock back over the coupler of the trailer for winter, securing with a rubberband/zip tie. Will this cause more harm than good due to condensation?  The guy that shrink wrapped covered the prop/motor but not front of the trailer. I fluid filmed the winch and coupler but want to try to keep in it working order better. Thanks

Unless you can be 100% sure water won't get in, I would leave it as is.  My opinion the fluid film you used is your best bet to keep the hitch working/looking new.  I have a few trailers, keep the moving parts lubed through out the year and you should have no problems down the road. 

I'd be concerned about condensation. I considered doing the same with mine but in the end just put a 5 gallon bucket (sideways) over the coupler. Keeps the snow and rain off but allows air flow.

 

Your trailer is designed to be out in the elements. Make it part of your spring rituals to lubricate the coupler, locks, boat buckles, winch, bearings, leaf spring pivots (if you have tandem axles), and what ever else that needs lubricating.