You do know that if you know your mpg then you can calculate the cost of a long trip and plan your budget. If you do not know if RV gets 20 or 3 or 12 then you just hope you are rich enough to not care if gas cost $1,000 or $500 or $2,500...
Is it cheaper to get a hotel and eat out for a trip or take the RV? the MPG people know...
Also, a sudden change in mpg might indicate issues that you can correct early or that will help the mechanic diagnose after a break-down.
I think these types of reasons are why people care, but does not mean that others should care about their mileage.