I don't have much evidence to support this, but I think this might be a reference to Robert Jordan, the main character of Ernest Hemingway's celebrated For Whom the Bell Tolls. Both are soldiers, to start. (I don't know anything else about either though.)
Also, there's this snippet from an interview with Crumley in The Austin Chronicle in 2001, about 40 years after this book was published:
Crumley taught part-time at various universities for 14 years before putting down roots in Missoula, Montana, a town that seems to breed and nurture writers the way Austin sprouts guitar players. "There are probably 70 or 80 working writers and another 200 who write," he says, "so I have all the advantages of being around a university without having to go up there and put up with their bullshit."
"But how did so many writers end up in Missoula?" I asked him.
"We never figured it out," he said. "We don't know. But this is where, you know, Hemingway's hero, Robert Jordan in For Whom the Bell Tolls, was headed. After the Spanish Civil War, he was gonna come up to Missoula and teach Spanish."
So, coming from Crumley, "Robert Jordon" could be a hat-tip to the character who inspired so many.
It's a tenuous connection at best, though.