It's going to depend on the kind of finish you have on your trailer.
Don't use Goof-off for goo removal. I've used Goof-off to remove paint and bathroom tub and sink sealants!
I wasn't certain about Goo-Gone, so, I tried some on an exterior water-base house paint can just now. With very little rubbing, it removed the paint also.
Since I've used baby oil to remove medical tape goo on skin I tried it on the paint can. To my surprise, even that started to remove the paint from the paint can!
Maybe your best bet would be soaking it with water with a lot of dish soap in it and using one of those "can't scratch" plastic scrubbers, very, very gently? Or, maybe careful but tedious scraping with your fingernails? Got a girlfriend or wife? Offer her a manicure and spa treatement in trade!
How about using a tarp and a tie-down rope to cover it next year?!