We have a 2016 Thor Synergy TT24 on a Sprinter chassis. Like everything else mileage depends on so many things. With two adults, full propane, 20 gallons of water, three cats, and "...a partridge in a pear tree" we can see anywhere from 14.4-16.8 mpg.
We came back from a 2,200 mile trip from WV, through VA, NC, SC, GA, FL; across Florida, and then back up through AL, TN, VA and back home to WV. Our best mileage came outside FL (VA/NC/SC/GA and then AL/TN/VA/WV at 15.5-16.2) and the worse mileage came while in FL (around 14.5) so perhaps there is more biodiesel blends in FL than the other states. I refuled primarily at Shell and BP stations and only in FL did I not see the "40" minimum cetane number on the pumps. I still like the Sprinter and have about 22,500 miles on the odometer since Jan. 2016.
Ed & Robin Lively
with Nina, Kitt, and Mickey
2016 Thor Synergy TT24 (a.k.a "Ming")