Your fuel mileage will vary just like anything else. The size of the engine, The transmission.
Cost again depends on the options
Tolls. Some coaches have a tag axle lift so in reality if the tag is not touching the road surface you pay for two. Heavy duty dump trucks do it all the time.
With a tag you can increase your GVW, Most tag axles unlock at slow speeds to give you 4 wheel steering while parking at a curb. I lifting tag also makes turning much easier since it changes the pivot point of the turn