I understand how you would be totally mortified. I come from a very wealthy family, and I used to feel sorry for "poor people". Now that I am "poor people", I realize there's nothing here to feel sorry for. I wouldn't apologize. I think that would make it worse. I live in an apartment, and if your son said that, I would not feel that you needed to apologize. If you did apologize, I would think that you thought that your life was so much better than mine just because you didn't live in an apartment.
There is a lot more to life and happiness than where a person lives, or how much money they have, etc. I'm happy with my daughter, I love her with all of my being, and we are working toward a better life--but that doesn't mean that anyone owes me any apologies for having a bigger place to live than I do.
Talking about it would almost be as bad (to me) as saying, I'm not racists because I have black friends. That's how I would feel if you said, I have friends who live in apartments and mobile homes, so I'm not judging you lol.
If you REALLY feel like you have to say something, then just use your own personal experience with growing up in apartments, and put it in an encouraging way. I'd say, I hope my son's comment didn't offend you. I don't know why he would say that, he knows my husband and I both grew up living in apartments...or something...I don't know really. If you know they are working towards something better, then say something encouraging about that. If they are happy where they are, then just say something nice about how you grew up. Just whatever you do, don't say something that makes it sound like you're above them, or that you feel sorry for them.
Good Luck!