Go to RVTrader.com and plug in your own specifications and see what people are asking within a hundred miles or so of your location. Private sellers are a better indication of value than dealers. Dealers build in negotiation room in their retail price.
NADA trailer guides list SOME fivers, but not all.
I guess consignment could work if there is a nearby high volume dealer. You need lots of lookers. Also read the consignment contract very carefully. Can he sell it out from under you for almost nothing? (That was a complaint on consignment car sales here in Central Florida.) What is the Commission? etc.
Individually or combination? Your call. Just remember that you probably don't want to sell the truck and then have no way to move the trailer that hasn't sold yet.