Attempting to fix a bad spot on the roof will cost you more time and money and patients that you'll want. Here's a very simple and easy solution.
Almost guaranteed to fail, also. I think most folks that have used expanding foam know about the drawbacks from trying to seal anything, the foam shrinks and nothing remains sealed. The video linked to in the previous post shows the correct way to replace rotten roof sheathing. Trying these hack half assed steps to circumvent the work is inviting just more work or, in this case, more leaks and destruction of the trailer.
My suggestion would be to do it right and replace the small soft spot with new sheathing. By the time you have the roof membrane pulled back, you are half-way done. Don't listen to these hackers with their lazy solutions.
