I don't know about street parking being illegal. Maybe in some communities it is, but I doubt that an ordinance like that would hold up in a lawsuit. Matter of fact I seem to remember reading years ago about someone that was ticketed and harassed by the local police and took it to court and ended up forcing the city involved into paying damages. It was long ago and I can't really remember the particulars, but I seem to remember that the court case went all the way up to something just south of the Supreme Court. Either way, if you and your vehicle are legal, and legally parked, I seriously doubt any ticket would stick.
Now when it comes to parking in urban areas, discretion is a must. I've been doing it for awhile now, and I have found that parking on main thoroughfares works real well, but you have to deal with the noise. I am not a big fan of McD's or some mall lots or places like that because that is where most vehicle theft occurs.
I also look for places that truckers like to stash their semis for the night. Not every inner city has a truck stop, but there is always a dead-end side street somewhere where they park and the police look the other way. You can find them, you just need to go look around the town's trucking terminals. They are usually all congregated in one part of town anyway, and the traffic that goes through those areas are usually workers at the plants and truckers, not gangs or other kinds of trouble makers. Also those areas usually have roving security, and the coppers patrol them regularly, and they are also well lit.
I work in South Philly and I actually park on a little side street about a mile from where I work, usually with about 3-4 semis on any given night. It is a wide, well lit street that is patrolled regularly, and has a 24 hour restaurant on the corner that is popular with the local constabulary, as well as the truckers and myself. The coppers patrol through there routinely and nothing ever happens. Even the lot lizard girls don't show their faces around there. Once or twice a local cop has stopped and asked after me when I was getting in and out of the vehicle, and I just tell him that I work here in S. Philly but live in Texas and this is where I park My RV at night. Since they have a detail of Philly cops that moonlight where I work, some of them already know me, and they keep an eye out for me when I'm there. I used to stay at the WM around there, but the truth is that the WM RV and semi-parking is in a desolate, trash strewn area that has a lot of undesirable activity going on around it. I feel safer parking where I park now.
One of the reasons I bought an RV was because I wanted to save on motel and rental car costs, which in the Northeast are spiraling out of sight. So, if I stay at a campground, it kind of defeats the purpose of me saving money on my travel expenses. If I don't use WMs, truck stops, or street parking, I might as well go back to hotels and rental cars.
Parking on the street isn't really all that dangerous. You need to learn to keep a low profile and be aware of your surroundings. I have never been vandalized, and the only time anyone has ever bothered me late at night, it was one of my co-workers that saw the RV parked and started banging on the side just yelling and fooling around. No big deal.
I do it in S. Philly, and right now I have been parked on a street in Washington DC since last Friday, albeit in front of a friends house. But I sleep in the vehicle every night, and get up in the morning with no hassles from neighbors, police, troublemakers or anyone else.