ER doesn't have many regular lower-class characters (Frank comes to mind, Pratt's real father, there are others, but I haven't watched it faithfully in a while), at least if you're only counting characters that appear every season or a majority of episodes.
But they usually do feature at least a few lower-class characters every episode, and the implications of being poor/working poor and the implications on health. The writers and medical advisors are very good about staying true-to-life for the actual medical aspects, including the demographics of people likely to be affected by various conditions and diseases.
Too bad next year is its last year on the air for new episodes.