I see this a little differently. First, it was YOUR trailer and there was no requirement to make repairs (unless it was being financed and they demanded the repair). Second, since no work was started, you don't owe anyone anything, unless there was a charge for the repair estimate or storage.
The dealer knew what he was getting, since it was there for repair. Most likely, the dealer deprecated the trade in value by the amount of the repair, or you missed something in your contract demanding you pay for the repair (highly unlikely).
Lastly, your insurance company still owes you for the repair. The damage devalued your trade in price. It's really no different than if you totaled the trailer. You no longer own it, but they still need to give you a fair market value.