Ours is *very* close. If not running the genny, within a few gallons.
There are two things to check. One is that the mileage is accurate. if the wheel size has been set incorrectly, your mileage (and speed!) will read low. The other is that the fuel metering measurement is accurate.
1976 Southwind 28', '96 Winnie 34WK,
2006 Tiffin Allegro Bus 40QDP