My cheapy, home-made, do-it yourself thoughts:
While a clear cover would allow your nice trailer to be seen while shielded from rain and snow and hail...is it possible it would let enough UV light
through to cause sun damage anyway? AND it seems most clear plastics are waterproof, meaning that any moisture under the cover and inside the trailer might remain, without sufficient air circulation, causing stinky mold.
Why clear? But if that's what you really want,
You can buy plastic sheeting in big rolls at almost any hardware/big box hardware store. The rolls fold out quite large, so you can use that if this is really important to you. Be sure to get it as thick (mils -- the thickness of the plastic) as possible or it may rip from projections on the trailer, wind, etc.. You can use clear tape to join large panels together, and also to pull it in underneath to keep it in place...
However it works out, hoping it goes very well for you,