By far, the MPGuino is the most easily tailored and available MPG monitor in existence.
Here's the OBDuino variant thread, which should at least have some merit or inspiration for you.
Also, great work on the boat-tail! The taper, sure, might be a bit sharp, but for what I know, as long as your bottom taper is less than 15 degrees, you're in the zone. Motorhomes, especially Class A, have always put me off for many reasons (I'm a bit too fond of tents and bicycle touring!), but I appreciate your efforts in ecomodding one.