The 250/350 are identical dimensions. I think the 450 may be same too, although it may have a wider front track to better match up with the dually config they are usually ordered with. I would bet any 1/2 ton pick-up you see with a slide in camper is probably at or over the GVWR. I have the 6.7l diesel, while the fuel economy isn't anything to get too excited about, it's as good or better than what I see people here post for their LRs and a lot of the SUVs (~16-17 in city driving, 18-19+ Hwy, this includes mountain passes and forest service roads)
I don't know why they would have based this vehicle on a 350 other than just because they could. You can pack a lot vittles in the back there. It would have a 37.5 gal fuel tank, so it'll give decent range. It's already got the aerodynamics of a brick so all of the roof rack and tent will eat into that quite a bit I'm sure. Don't know what the B-100 diesel and or Algae diesel would do to mileage. Mine can run on B-20, the few times I have it seemed to be a little lower and caused more regenerations (the new diesels have a soot filter in the exhaust that periodically gets cleaned by increasing the exhaust temperature to burn it off. They do this by pumping raw fuel into the system - I won't get on my soap box about how much this wastes by dropping over all fuel mileage). Bio-diesel can absorb more water which is not good for the high pressure fuel pump (runs at up 30k lbs of pressure to inject fuel), any kind of moisture will not do these pumps any good, when they let go, they grenade the entire fuel system to the tune of about $20k. Pays to be watchful of fuel quality which is where most bio can be suspect. Oh, and they can't run on high sulpher fuel so no taking to Mexico or over seas with modifications