About Sportmobile vs RoadTrek or PleasureWay, they are building different things, so it depends on what you mean by "quality." Sportmobile builds camping vans and purpose built vans to suit your business or hobby needs. Build quality is excellent, finish is great, but materials are functional rather than screaming "luxury motorhome." Similar to what you might expect from Winnebago but maybe better put together. RoadTrek, PleasureWay and Airstream are building small luxury motorhomes for people seeking something resembling a tiny luxury motorcoach. That's partly why a custom built Sportmobile might be $20K to $40K lower than a new B off a dealer's lot; the other part is that the dealer still needs some margin to stay in business, while Sportmobile is factory sales.
MPG. I don't know what a Sprinter really gets. DIN figures for the old 2.7 five cylinder translated to 22 MPG for the high top. DIN MPG for Sprinters with 2.1 and 2.2 liter diesels with manual transmissions were better, but we got only the I-5 with automatic. In the second generation, DIN MPGs for the 3.0 V6 sold in the USA have not been any better, but DIN MPG does not translate well to EPA tests or U.S. driving conditions (e.g. 80 KPH highway speeds).
You need some Sprinter people to tell you what they get.
My 2013 E-350 van with 5.4 V-8 has averaged 14.8 MPG over 12,000 miles, but I'm just breaking it in, that's not enough distance to get a good average. Most of my driving is either stop and go around town (as low as 12 MPG in summer) or road tripping. Best MPG performance has been traveling 2-lane highways in the winter, no A/C running, getting 20-21 MPG several fillups in sucession. On the other hand, filling up with E-85 while going through the Midwest I saw a drop of 2 MPG, which still pays off if E-85 is more than $0.60 cheaper than E-10 regular.
However most of the year, I am running both A/Cs full blast to keep a van full of passengers cool, and that eats up a lot of gas no matter how slowly you might be moving, so it really cuts into around town MPG.
Ford has a 4.6 V8 offered as base engine for E-150 and E-250 that is supposed to be 1-2 MPG better, average driving. Indeed, the only real difference between E-250 and E-350 at the end of production was that the E-250 offered the 4.6 V-8, while the 5.4 was base engine in the E-350.
FWIW, there is no longer a Ford V-8 van, the E-series has been replaced by the fifth generation Transit which offers only V-6 gas engines and a five-cylinder diesel for the U.S.
Chevrolet, it depends on which of four engines is installed, and which transmission you get. The 5.3 (the V8 for 1500, base is a six) with 4-speed does about the same as a Ford. The 4.8 (base engine for heavier vans) does a little better. If you get a big enough van to need the 6.0, it will get about the same as the Ford 5.4 with a 4 speed, but will do 2-3 mpg better around town with a six speed (recent models).
M-B diesel is going to consistently do a lot better than this, and how it does will depend on how high a top you buy.
Unless you go way back, you won't find a V-8 Dodge. The Dodge van got replaced by the Sprinter.
The Dodge van was built in 2003, design dating to 1994. V8s offered were 5.2 and 5.9 Magnum small blocks, with common rail fuel injection. These are comparable to the engines Ford abandoned in favor of the modular or Triton V-8s in 1997, and those Chevrolet abandoned in favor MPI Generation III (and since 2007, Generation IV) small-blocks.
If you are thinking about buying something built on an old Dodge van, buy no earlier than 1994 and look for the 4-speed overdrive, not all vans had overdrive.
Current van from the successor Fiat-Chrysler America is the Ram Promaster, an Americanized, North America-built Fiat Ducato. Engine offerings here are the 3.6 Pentastar V-8 from Chrysler minivans and sedans, and a Fiat four cylinder diesel, 3.0 liters, with a self-shifting six-speed transmission. The diesel probably does about the same as a Sprinter of similar height and weight, although the 2.2 liter diesels used in Europe get about 20% better MPG.
I suspect what you are looking at, modern engines at least, will be 20-24
MPG diesel, unless the RamPromaster turns out to do better, versus 14-16 MPG with a gas engine of 240-300 HP, whether it is a 3.7 V6 or a 6.0 V8. Older V8 engines will do much worse, 8-12 MPG was not unusual in the late 1970s through the 1980s.