I recently moved up from a 2006 Chateau Class C built on a Ford E-450 with the 6.8 V-10 platform to a new Seneca built on the Freightliner M2 chassis with the Cummins diesel. While the Ford served us very well (never a single breakdown, only routine maintenance) the difference between the Ford and Freightliner capabilities is quite significant. 14,050 lbs. GVWR on our former Ford, 27,000 lbs. on the Freightliner. We would get 6.5 - 8 MPG on the Ford over the 8 years we had it, 10.8 MPG on the first trip we took with the Seneca with less than 1,000 miles on the engine! That was even towing a SUV 1,500+ lbs. heavier than the Focus we towed with the Ford.
We looked hard at the Thor medium-duty products since I am a die-hard Ford fan, but came away convinced it wasn't for us. Research showed that even though the Thor F-550 units have a 19,500 lb. GVWR; by the time you fill it with fuel and water there isn't a lot of reserve for people and cargo. With full fuel, water, and propane my Seneca still has 4,000+ lbs. of capacity for people and "stuff".
Another factor was serviceability of the engine. No comparison between the Ford F-550 cutaway pickup compared to the Freightliner M2, where you tilt up the entire front end exposing almost everything on the engine. Since I service my own unit that difference was another factor I weighed in my purchase decision. And if you research the Ford F-550 diesel online you hear some horror stories regarding the seemingly fragile injector pump with many customers eating huge bills to replace the pump when Ford denies warranty coverage claiming "contaminated fuel". Not that it cannot happen on any diesel, it just seems more common on the newer Ford Scorpion engines.
I loved my Ford E-450, and they probably have gotten even better. But if you want more cargo and towing capacity than the van chassis offers, it was not much of a contest between the Freightliner and the Ford F-550 chassis.