I like Goof-Off, but it is nasty nasty stuff. I would try it on the skirting somewhere out of sight and that will not matter if the waterproofing is compromised. My gut feeling is it will remove the vinyl coating. As for a cleaning agent, I would consider using a product called “Spray Power” by Crown. I bought it at Menards about $6.50/gal and I have had very good luck with it. I have not used it on my canvas yet, but my parents have and they said it cleaned their awning effortlessly. But did they have to deal with pitch???