Seems to me a person could basically build a tent structure to house the trailer. Depending on the shape of the trailer (yours looks prime for this) it would be cheap to build some small trusses that span the width of the trailer. One every two feet would be perfect (works for the house, should work for the trailer. Place the trusses on top of the trailer and screw in a couple of boards diagonally to keep them from tipping. Then drape a tarp over the whole thing and pull the sides out and stake them with something pretty strong. The farther you pull them out, and the longer you made the eaves, the more ventilation you will get. I would guess a person could do this for under $100, the cost of the tarp being the majority. If you wanted to get fancy you could put in a door so you could access the inside during the winter. Or you could do a smaller tarp so the sides would be open and air could easily flow through.
In the spring, detach the diagonals and stack the trusses for easy storing.