Oh he certainly wants to serve someone else.
When he's captured by the Brotherhood, he's going north. He tells them that he was going "to feed a bunch of peasants and their poxy welps" with his tourney winnings. They disregard that.
Later of course, he captures Arya for "ransom." In actuality, he he was planning on returning her to her family in the hopes the Robb would accept his service, and perhaps make him a lordling. Seems to me he actually did want to use his gold to feed some peasants. Arya says this will never happen--Sandor's alternative plan is to get gold as a reward instead.
After Arya leaves him, he's in a totally shit spot. No family, no friends, lame, dead broke, everyone (even villagers in the Vale) recognizes him, and everyone wants his head for deserting the Lannisters and/or for atrocities at Saltpans.
Remember he's a cynic and an atheist. He's in his late twenties and a famously competent warrior. He likes fighting and drinking and sex just like any other fighter in the series (though he did use those things as an outlet for his emotional problems, no doubt). I have a hard time seeing a guy with those characteristics turning into a monk. Last we see of him, he's clearing away dirty dishes. This does not seem to me like a happy future for Sandor, but he doesn't have many options in life.
Notice how when we first see him he almost hits the party with a shovelful of dirt, and when Dog comes up he drops his shovel immediately. I'm not going by much here, but you could read that the gravedigger passive aggressively resenting his work.
Also notice the inclusion of the story of castrating Stranger, the warhorse. The brothers name him "Driftwood," and want to castrate him to make him docile so they can train him to work the land, but Stranger rips an ear of the monk who tries it. Funny anecdote or symbol of what's going on with Stranger's master?
Personally, I think Sandor is coming back. He's mentioned a lot for a character who's dead and gone. In AFFC, Jaime makes at least 4 references to Sandor's battle prowess, and Cersei thinks of him also. In ADWD, Brienne tells Jaime that Sandor has Sansa. My personal unsubstantiated pet theory is that Sandor will recover sufficiently on the Isle, decide that he can't take being a monk, and set out to kill his impersonator (Lem), during which he will run into Brienne and Jaime. The confusion would be hilarious, Jaime would be spared, they would be pretty formidable together, and all three would be interested in rescuing Sansa. Jaime and Sandor have so much in common... disenchanted with knighthood, cynical, witty, Sandor guarded Joffrey and so obviously he and Jaime are very familiar with one another (Sandor may even know of the twincest), and now they're both injured as well. Additionally, Jaime is the only Lannister sibling that Sandor doesn't talk smack about (despite the familiarity), and Jaime has a hard time believing Sandor is responsible for Saltpans.