Anywhere between 4 and 12 mpg (US). The lower mileage tends to come from gas/petrol engines -- the Ford V10 is popular. As well as smaller diesels that are towing another car (A "dinghy") up hills. The higher mileage is usually from Mercedes Sprinter based RVs, and Class 7 & 8 truck conversions.
Costs when driving are high, because of the fuel expense. But once you're parked in a campground (or even in the street like in the story) the costs are pretty low -- $200-400 a month (or $20-50 for a daily rate) for a campground. $50-200 to refill the propane tank. And $20-40 to empty the black water tank at a dumping station. Some campgrounds charge for electricity (20, 30, or 50 amp connection). Internet can be had over a parks WiFi, satellite, or a MiFi device (factoid: a 50GB plan on AT&T is $300/month), or if you're monthly, a campground may allow TimeWarner or Comcast to run a line to your rig.
Costs when driving are high, because of the fuel expense. But once you're parked in a campground (or even in the street like in the story) the costs are pretty low -- $200-400 a month (or $20-50 for a daily rate) for a campground. $50-200 to refill the propane tank. And $20-40 to empty the black water tank at a dumping station. Some campgrounds charge for electricity (20, 30, or 50 amp connection). Internet can be had over a parks WiFi, satellite, or a MiFi device (factoid: a 50GB plan on AT&T is $300/month), or if you're monthly, a campground may allow TimeWarner or Comcast to run a line to your rig.