It wouldn't be honest to sell the trailer to the next gullible person that doesn't know about the roof leak.
I think I would rather own a 7K trailer that I have 10K into rather than a 7K trailer with a roof leak and damage.
Or, I would rather own a 7K trailer that I have 10K into rather than sell it for 4K (so someone else can fix it) and buy another 7K trailer. After taxes and hassles, you're probably back to where you are now and not any better off financially.
Call it a 3K mistake, fix it, and move on.
2014 Primetime Crusader 296BHS
2015 GMC 2500HD Denali