I am in the middle of working on a post-apocalyptic game so I had to work out some of this stuff already. Here's what I found. I think it is a matter of power and radiation. If you go up into coal country I am sure that you can keep those power plants running throughout the nuclear winter, same with gas country. Coal mines and oil wells are common enough that plenty should survive a war. Hell downtown LA has oil rigs disguised inside of plenty of buildings so they could find a way to refine it and power generators. So you could have groups built around those pumps and the what they do with it.
Yes, some of these groups could be heavily radiated if the bombs feel near by. They may die young but still produce power that can be used by other groups.
So once you have power you can build greenhouses and produce food. So if the story takes place early enough then there may not be any major communities. Two or three years out as they get the food production stabilized the communities surrounding it will begin to prosper.
The locations will most likely run by a warlord or some other kind of armed organization that can keep the locations safe enough for work to get done.
Check out these links with articles on indoor farming. New Jersey already has one in an old factory. Metal and concrete structures would be great at block out radiation.
Clean water would be the next hurdle If that can be found along with power and the people to put it all together then you could have a few clean healthy towns. If they get big enough they may try to expand out and take over other communities. That is if this takes place far enough in the future for the communities to have a large population.
Radiation would be really bad for any kind of community. Children will have too many debilitating birth defects, many mental to allow the community to survive so if power is in a radiation zone then maybe only the old are sent to work there since they would never live long enough to be debilitated by it and, of course, would not be having kids so that isn't a problem.
Now if you are talking about scavenging then areas that are not hit would run out of all food pretty quickly as everyone hordes any food they can reach. So anyone that came later would have to break into houses to find caches or raid them if people are still there. The only place they could find untouched food would be in the ruins of the nuked cities.
Maybe an organized team could send teams in to retrieve salvageable food. The dishonest may go and grab anything they can find and sell it as normal foodstuff. A Geiger counter is a must have item when shopping.
So in early days lots of people trying to gather the stuff to survive and build some kind of life support and in later days communities emerging around greenhouse farms and finally cities built around those same communities and even expanding to take over old areas.