Although, I admit, I'm still questioning whether Robb survived the RW by warging first into Grey Wind, then Reynald. One needs only inspect the events leading up to Reynald plunging into the waters of the Green Fork. It's my theory that Robb was perhaps warging Grey Wind with his final words, much like Jon did with Ghost. Seeing that Grey Wind was tied up and could not escape, he then warged into Reynald who had surrendered previously. Reynald had a sudden change of heart and cut Grey Wind's bonds. However, it was too late for the wolf and Robb/Reynald, wounded himself, staggered off of the bridge. I will remind you that we didn't see him die or see his body washed ashore.
Robb may be biding his time somewhere, waiting for the perfect moment to prove who he really is, and take revenge upon his killers.
That's really a far stretched man to be true