For independent inter-city travel and in the city zipping around, nothing beats those fast little cyl hatcback cars, but the downside is they are usually too small to comfortably sleep in, thus really living out of one requires having places to stay with people you know, or throwing money on the hotel rooms.

Here are some survival basics for the city:

Water - Cities tend to pride themselves on the quality of their tapwater, and in 2011, most cities have decent tapwater. Numerous studies show that city tapwater is on average, going to have fewer contaminants than most farmland well water. Why? Well water often has pesticide runoff and minerals that are bad for you in the higher concentrations, and the quality of well water is nearly impossible to keep consistent. Well water around livestock areas commonly has salmonella and other bacteria that live around animal waste.

Food: while food is not produced in many cities, it is processed and prepared in most cities. This includes bread factories, meat processing plants, packaging plants and trans-shipment facilities. Sure, some people call it "looting" but spoilage vs putting stuff to use is also an issue. In non-SHTF scenarios, that's why they have food banks, places that take food which is near spoiling and give it away, and quite often even the food banks throw a lot away. In SHTF, it gets taken out anyway. Newer disaster plans with some of the chain stores around here are to have armed guards at the stores, but also the stores giving away disaster ration packages of supplies which get signed out as charitable giving, thus when things recover, the spoilage material they were giving away anyway is an orderly tax writeoff. I personally think market forces alone will mean food is always available in cities, but of course costs would vary. Farmers don't want food rotting in the field, city people want to eat, farmers probably need (or at least want) fuel and manufactured goods.

Sewage/facilities: These tend to be engineered to remain operational for quite some time, but then that would depend a lot on the scale of any particular disaster. In social breakdown circumstances, a lot of this stuff can be made operational on a pirate scale since it stays turned on for government buildings and the wealthy. Apart from keeping yourself presentable enough to get into downtown buildings and using public restrooms, understand also that many now include showers which are mandated in newer building codes to accommodate bicycle commuters. In suburban areas, making use of these facilities may mean pirating some water main connections going into abandoned buildings, opening up short term utility accounts under assumed identities, or otherwise faking access. These activities have their own risks, but are not necessarily as bad as the risks you run of infection and uncleanliness with remote outpost latrines and outhouses.

Electricity: Cities without power get dark really fast, mainly because the density of construction prevents natural moon and star light from getting through, and even twilight/sunset does not last as long in cities as in the open country (although mountains can influence that). The plus side, cities tend to be home of utility workers and companies with a very high economic incentive for keeping the power turned on. If you do obtain a generator, it is not particularly difficult to obtain modest amounts of fuel that it would need to keep running. Siphoning from unused cars being a big source. Cities also tend to have enough fairly open rooftop space here and there to set up solar panels on a modest scale with nominal off grid capability for hold-outs.

Heat: Fact of the matter is, buildings are easier to heat then tents no matter how you ad it up, even if you have to knock holes in floors, ceilings and walls to retrofit some old buildings with improvised wood burning stoves or dust off old coal based heating systems. If you have electricity in a building, then you can carry electric space heaters into it. Adding insulation or making subdivided rooms inside an existing building is an art form in the Northern cities.

Expert personnel: If you need a neurosurgeon, you are quite likely NOT going to be able to find one in the boonies, in fact, skilled country doctors are so rare now that cities are pretty much where the medical facilities are, with some notable exceptions. That is not to say the country lacks skilled people in some fields of expertise, but the nature of the rural area often limits that to the types of expertise they use on a regular basis. Mechanics, woodcutters, farmers. Not a whole lot beyond that. Cost effective local labor is a matter of argument. I just personally have had better luck hiring semi skilled English speaking general labor in the cities. Other people's mileage varies on that. Another thing, if you want to recruit people for various tasks, you are slightly more likely to be able to recruit them with less effort in the city.

Garbage: Maybe not exactly a militia subject but something to consider. Cities require garbage removal to be tidy, and city managers have known that for hundreds of years now. Country people on the other hand, don't always entirely comprehend the concept. Even without formalized garbage services, people who decide to be clean can be clean, and in cities with limited resources, that means establishing, organizing and enforcing dump lots. These are the abandoned lots and burned out buildings which will become garbage dumps until the main post-SHTF clean-up happens. Real world, the dump-lot would occasionally get re-burned if major haul-offs cannot be arranged. In the country, people tend to burn their trash more often than not, but then there is crap that always tends to get hoarded and piled up because someone thinks it will be useful someday.

Security: Cities are what Tzun Tzu called "heavy ground" meaning that they take a lot of people to really control any given area and in reality multiple groups and individuals control different areas. This also would relate to Machiavelli's concept of principalities, in that the modern city contains numerous "principalities" and anyone who aspires to or has any political power will want to be flexing it in the city. That means power issues over some things which are never much of a question out in the country. That and the normalcy of total strangers commonly being in close proximity to your vehicles and stuff, and the fact that government power usually has strong representation in cities although even that may be localized and time sensitive, IE, even the streets immediately outside of courthouses and police stations can change their character from mundane to sinister after dark. Then you have property and turf issues everywhere, and I mean everywhere, so even parking becomes an issue which can challenge your ownership of your vehicles, let alone occupation of any semi-permanent space which could challenge the ownership of your goods without sufficient resources dedicated to the security issue.