1.) With all due respect, RS's guides are not the bible, are simply one point of view, as legitimate as anyone of ours in the forum, local or visitor.
2.) It's very true that unfortunately some Americans do travel abroad with an arrogant attitude and believe 'everybody is out to get them'. Obviously that's fruit of their profound ignorance and, with that attitude, I wonder why they travel at all in the first place. These are those that quite often bring back home bad experiences because of their bad omen. Fortunately it's a tiny minority though.
3.) Crime do exists in Europe, as it does everywhere else, and it can be as violent as in the suburbs of many US cities, so without being paranoid it's best not to fall into a false sense of security either. Having said that, as it happens in the US, crime (and violent crime) is less likely -and even almost inexistant- in rural areas than in it's in more urban areas. Yet it's simply absurd though to pin it to "unemployed" people as stats show that most crime, even minor crime, is perpetrated by 'career criminals' (as in 'thugs') many of which might have regular jobs too. So unemployment does not make a significant (as in 'major') impact on crime, at least that's not what stats show over here in Spain. Right now unemployment is at an all time high, if that was true, we should be robbing at killing each other much like in The Purge... and we're not, we continue to be one of the safest countries in Europe and far safer than say the US :)) Furthermore, car break ins are crimes of opportunity so criminals don't necessarily have to be local, albeit many are, yet you'd be surprised the scores of tourists that are arrested every year in our coastal cities for committing crimes -for some it's seems to be their way to finance their holiday!
4.) As a general rule, it's unwise to leave stuff inside the car in most European big cities (Barcelona, Milan, Marseille, Rome, Seville, Amsterdam, Paris, Athens...). Full stop. If you have to, leave it only in the trunk and, if you can, try to avoid being seen leaving it there. Also, if you have to leave valuables in your car (in your trunk that is!) avoid parking in solitary streets and back-alleys. Use parking lots or underground parking whenever you can as break-ins there are less likely. Yet in short, AVOID leaving stuff in your car.
5.) There's absolutely NO WAY to distinguish a rental from a regular car as rental companies do use the same exact models that you'll find among many Europeans. Note however that some rental companies identify their cars with a sticker (ie GoldCar among many others) which, makes good advertising for the company but it's really a bad idea for you since it announces to potential burglars that "you're not local".