If you are looking to put trailer on site for a season, get it in place, cut a tarp the perfect size, use some lath around the perimeter to hold the tarp and just use it....I am afraid a trailer like yours in its condition will not have much value in the sale or trade dept.......the tarp will buy you some time, then when and if you decide, down the road you can do a roof the right way.....do a search for your trailer, see how much they are going for, probably not much more than the cost of a new roof......good luck..