Certainly you have identified one of the major themes of this novel as it depicts the infamous "jazz age" of the 1920s that only really the wealthy were able to enjoy in all its hedonistic decadence. A great place to start if you are trying to identify quotes that support the theme of the hollowness of the upper-class is Chapter Three, that details the first of Gatsby's parties that Nick attends. Key to this description is Nick's opinions of these wealthy upper-class individuals:
Once there they were introduced by somebody who knew Gatsby, and after that they conducted themselves according to the rules of behaviour associated with an amusement park.
Clearly this indicates the riotous behaviour that the guests (half of whom invited themselves) engaged in.
You also might like to consider Daisy's famous quote in Chapter Seven:
"What'll we do with ourselves this afternoon?" cried Daisy, "and the day after that, and the next thirty years?"
Clearly this points towards her own sense of emptiness and futility in her life as she tries to fill it in whatever way she can. According to Nick, of course, upper-class individuals like her only "smashed up things and creatures and then retreated back into their money or their vast carelessness..." and this again points towards the essential hollowness at the heart of the upper-class that Fitzgerald so competently dissects.
In The Great Gatsby, the "hollowness of the upper class" can be seen in the attitudes and behaviors of individual characters.
"Get some more ice and mineral water, Myrtle, before everybody goes to sleep."
"I told that boy about the ice." Myrtle raised her eyebrows in despair at the shiftlessness of the lower orders. "These people! You have to keep after them all the time."
In the above quote, Myrtle's reply to Tom Buchanan highlights her condescending attitude toward people of "the lower orders." Myrtle is the kind of woman who can't see past outward appearances; she is preoccupied with status and wealth.
In the quote, Myrtle tries to distinguish herself from the "lower orders" in order to reinforce her supposedly privileged position as Tom Buchanan's mistress. Tom, after all, is a very powerful and wealthy man. Myrtle's words exemplify the "hollowness" of her worldview; she's willing to commit adultery in order to attach herself to wealth and privilege. She's also quite willing to entertain an attitude of disdain toward those who fail to meet her expectations. Myrtle's attitude is indicative of that held by the "hollow" upper-class community she insists on being a part of.
For Daisy was young and her artificial world was redolent of orchids and pleasant, cheerful snobbery and orchestras which set the rhythm of the year, summing up the sadness and suggestiveness of life in new tunes.
She wanted her life shaped now, immediately and the decision must be made by some force of love, of money, of unquestionable practicality that was close at hand. That force took shape in the middle of spring with the arrival of Tom Buchanan. There was a wholesome bulkiness about his person and his position, and Daisy was flattered. Doubtless there was a certain struggle and a certain relief. The letter reached Gatsby while he was still at Oxford.
In the above quotes, we discover the real reasons Daisy married Tom Buchanan: she chose Tom (and not Gatsby) because of "unquestionable practicality." In the text, we are told that Daisy gets cold feet the day before the wedding. Tom has purportedly given her a string of pearls worth three hundred and fifty thousand dollars, and all she wants to do is return them. Daisy's infatuation with Gatsby continues to haunt her, but her upbringing demands a practical decision from her. In the end, Daisy marries Tom.
Daisy and Myrtle are two women who base their actions on a "hollow" life philosophy. Both are attracted to Tom because of their primal and practical instincts; the need to transcend irrelevance is so strong that authenticity must be eclipsed by cold practicality. Their actions highlight the superficiality of the upper-class worldview, where status and outward appearances take precedence over truth.