You either did or did not trade the trailer as-is and either did or did not agree to give the dealer the insurance proceeds. You state you instigated a claim and signed a document to allow the dealer to be paid for the repair directly from the insurance company. That could certainly lead one to believe the dealer has a reasonable expectation to receive the insurance proceeds.
You obviously knew you had insurance, knew the damage was $5900, knew it was potentially a total, knew the book value if it was a total, and knew the insurance company was obligated to pay. Yet, you traded the unit, accepted a $2500 value, and gave them the title. What were your expectations? If you thought you were getting the $2500 and the insurance proceeds, good for you and I hope you have that in writing. If you decided post transaction that you don't like the deal you made and you are searching for a way back in...shame on you.