My husband does automotive paint repair on cars/trucks/vans. His suggestion is to take the travel trailer to the automotive paint supply store and they'll use a special camera to take a picture of the paint and match it from that. It may not be a 100% match, but he said that it'll be so close that you won't be able to tell the difference.
He says that not all paint supply stores have that camera, so you should call before you go. Also, he said that most body shops that the camera, though I don't know how willing they would be to help if they aren't going to get your business.
Hope that helps!!
Kari and the gang
2013 Salem 30KQBSS
Total nights camped in 2012 - 23