He may mean UK gallons. I remember a Canadian member on DP getting all kinds of butthurt that the dash used US gallons.
If he is calculating as UK gallons, then in US gallons, he gets 20.8 MPG in his truck, which is still a bit of a stretch, but can happen with the right tires and flat ground. (I've done 19.8 MPG with 285s at 70 MPH, bet I could hit 20+ MPG with 245s and no wind.) Alternatively, the tire size could have been entered wrong on his programmer, which would throw off his speed and mileage calculations.
As for the oil loss, it has to be going somewhere. A leak down test might have some answers, but it might not. If it isn't leaking outside the engine somewhere, then it is either going through the PCV or through the combustion chamber (rings, valve seals).
It sounds like your PCV reroute doesn't have a catch can. You might consider adding one temporarily to see if that is what is happening. Oil doesn't always leave a residue in the tube, especially if there is a lot of air/vapor going through as well.