Is it possible for a PhD student to survive in the USA only on TA/RA money?
Agnostic of institution, each of which has its own set of policies governing TA/RA pay (more or less depending on the institution), a short answer is yes.  There are also, of course, different costs of living depending on where your institution is located (e.g., the cost of living in New York is much higher than in, say, the midwest). In any case, you will be paid a livable wage, but don't expect to be living any sort of lavish lifestyle.

Personal experience: I've been living off of TA/RA pay for the past 3 years. The pay offered by the R1 institution I attend and work for is located in the Midwest and, relative to other schools in our conference, offers one of the best stipends. As such, I've had enough money to not only live, but support my hobbies and enjoy a [non-extravagent] leisurely life during my non-working, non-academic hours (of which there are not usually a ton haha).