In dash MPG calculators are notoriously in-accurate. I don't have one and would not use it if I did. Simple spreadsheet with columns for all the info.
My Winnebago View shows 17.539 mpg over 17 fillups and about 4000 miles. Those are the numbers since I bought it.
My '91 Ranger pickup now has 296K+ miles on it and I started the spread sheet in late 2011. Since the beginning of the spreadsheet, has put on 34,133 miles, and 121 fillups (it is basically an extra vehicle) and averages 24.559 mpg which gets down to 20 mpg on tanks where I pull the 5x10 lawn type trailer. This thing just about cannot get out of its own way.
My '04 Pontiac Vibe was the reason for the spreadsheet. It gets individual tank full calculations all over the map, because it is difficult to consistently fill it, so from when I purchased it, I have driven it 117,605 miles with 403 fillups and average 32.280 mpg.
You want numbers, a spreadsheet and diligence in keeping numbers and receipts is the way to go.
Charles
'03 Ram 2500 CTD, 5.9HO six speed, PacBrake Exh Brake, std cab, long bed, Leer top and 2008 Bigfoot 25B21RB.. previously (both gone) 2008 Thor/Dutchman Freedom Spirit 180 & 2007 Winnebago View 23H Motorhome.