New Traveler need advice
1- The older a unit is, the more likely to have problems. Much more likely than the equivalent car, as there are lots more systems. Gas engines seem to wear out quickly (a lot of the ones I was looking at with 50K or more miles had 'rebuilt' engines). Diesel engines last a lot longer with proper maintenance. Everything else can wear out or break or be damaged, largely at random but also based on how it was used and maintained. And the cost to repair something will usually be higher than fixing the same thing in a car or house, because not everyone can work on them and the parts tend to be more expensive.
If you can get an extended warranty for it, fine, but be careful, as many of them aren't worth the paper they are printed on. Try for an 'exclusionary' policy (everything is covered EXCEPT for what is listed) rather than an 'inclusionary' policy (ONLY what is listed is covered). Also, make sure you follow their requirements for maintenance and using the policy to the letter, or they will usually refuse to pay.
2- Insurance depends on the unit and the drivers and the coverages you select. For liability only, probably less than insuring an automobile. If you have a good agent for your car, see what they can do for the RV. But get quotes from some RV insurance specialists too. The insurance company I have been with for 20 years, no claims, all autos and home, wanted twice what I ended up paying for a better policy from Good Sam's (GMAC).
3- Depends on the weight, type of fuel, engine, speed and driving style. In most cases, it should be 4 to 10 MPG for large motorhomes, and 8 to 20 MPG for class B (van) campers (but you probably would not be happy living in one of these for any length of time). You ought to be able to get a 20% improvement in gas mileage if you go 55MPH rather than 75MPH.
4- Usually if you stay in a supermarket or on the street, you are subject to harassment from the local officials and private parties. WalMarts (and some other chains) can be a good choice except where prohibited by local ordinance (there will be signs prohibiting it). Rest stops are a possibility in many states, just be aware that criminals sometimes 'hunt' there. Truck stops and travel plazas are often a good choice if you can coexist well with the truckers who use them. Wherever you intend to stay, check with the owner/manager beforehand. Buy something from them, and leave it in better shape than you found it.
Also, check out Passport America, which provides 1/2 price camping at member parks. Generally I go between Arizona and Illinois, and usually I can find a PA site near where I want to be, and probably average $12 - $14 a night. Alternatively, a lot of parks have significant discounts for 'long term' stays.
5- I don't like AAA. In order to cover a RV, you have to buy their extended support and then buy the RV rider. Well over $100/year, and still subject to towing limits I think. Good Sam and a few others have a better plan, for under $100/year (you can usally get it at a discount).