You will need thousands of miles to get an accurate mileage reading if you're just going by mileage/fuel used. I really never worried about it as it is what it is. When towing the '05 Odyssey behind the Magna the VMS shows an average of 7.1 mpg and that includes Tehachapi, the Grapevine and sitting in barely moving traffic after an accident. For an almost 50,000# rig towing another 4,500 that's not bad.
2009 45' Magna 630 w/Cummins ISX 650 HP/1950 Lbs Ft
Charter Good Sam Lifetime Member, FMCA, SKP
RV'ing since 1957, NRA Benefactor Life, towing '05 Odyssey