[DK on the road]

I calculate the old fashion way, Miles divided gallons purchased each refill. Then an average over about period say 30k miles. Over the last 30k I've avgeraged 5.6 MPG at about 63 to 66 mph. I've gotten as high as 7 and as low as 4.9. The 4.9 was a 75+ mph run on a tank of fuel. Ps tires are rated at 75 mph, so not recommended.

I have a C15 625hp with the 12sp ZF transmission. My generator runs about 20% of the time I travel. I travel mostly in the Sierra's, Rocky mountains etc. 25% east of the Rocky's. I pull an "V" nosed enclosed trailer with a car in it at 10K , or Ford Diesel F350 with a motorcycle in the back, at about 7k lbs. I hardly drive without a trailer or truck.  

My fuel prices on the west coast run about $3.95 to $4.25. My point to this is, My cost are higher when I travel, per mile than a midwest, traveler at $2.40 per gallon. So it all depends on where you drive and How you drive

As a side note, my last diesel pusher was 10' shorter, 1/2 the weight and 1/2 the horse power, pulling the same loads. It got about 8.3 mpg average. It was just a bit slower climbing 6% grades, 42mph vs 52 mph in the Newell. That coach wouldn't last for 400K like I hope a Newell will.