1. Hookups (electric, cable, water) are included in the cost of the campsite. To get an idea how much extra they charge, look at the difference between no hookup site (tent sites) and full hookup sites.
2. Walmart and Cracker Barrel parking lots are free. BLM (federal land) is usually free. National Parks and state parks cost but are usually in the $10-30 a night range. Campgrounds a little more and RV resorts can cost 75-150 a night.
3. With a full water tank and enough DC power to run a pump, you can shower in a rest area.
(what happened to 4 & 5?)
6. No. The best mileage you can hope for is in a smaller class C which may get 17 mpg or so.
Gas is going to be your biggest expense. You can join Good Sam and get a Pilot/Flying J credit card and get a few cents off a gallon. The good news is gas prices are dropping. Diesel is still expensive. Next expense is regular maintenance, which, if you don't do, will be replaced by the expense of fixing things when and where they break.
Seeing that you're just starting to think about this, I suggest buying something old and cheap but in descent shape to help you sort things out.