My van (2014 Mercedes Benz 3500, 170 ext, dually, 35,900 miles) was delivered today and I got some real measurements - and was able to take a closer look at the interior.
In great news, discovered today that this guy is a 5-ton beast that can carry slightly over 12,000 pounds vehicle + load. (There had been some confusion because certain models in Europe were never released in the U.S. and also Euro models have two VINs - which didn’t work quite right in the MB Vin decoder websites).
So, the buildable space in the back is 16’ + a few inches. The dealer told me I should “cut out” a step at the sliding door…but, since there is no passenger seat - I think I’d rather not cut out the step and have a continuous surface across the van and not lose those 6"-8" of floor space. The right passenger door is literally 6 inches away - with plenty of room - so, don’t see the point in cutting up the floor. Without the passenger seat - the “foyer” is fine… 
The skylights/hatches are 40.5" x 22" external rim to rim. They can open - the dealer was able to do that - but it’s been a while since they were routinely opened - and I can’t figure out how to open them. I am trying to figure out from their website whether they can be retrofitted with fans. There are black handles on the short ends - but I can’t find a lock or mechanism that lets you open them.
I thought the wood on the walls and ceiling was too cheap and flimsy to leave on the walls and ceiling - and that they would need to be pulled out. But after looking at them up close - I actually think the walls are going to work.The van is only 66" wide, down from the standard 70.5 - but that’s ok. There’s obviously space between the van exterior wall and the wood and the wood is better quality than I thought. The “waviness” in the pictures was due to a poor paint job, not to the wood itself. It’s not the absolute best in terms of ply - but I think it is sturdy enough to serve as a base with some appropriate reinforcement. Only concern is rust - what if something is lurking behind the walls???
Question about the white box that all Sprinter seats seem to “sit” on: From what I could see from a peek in - it looks like some sort of fuse box or . . .???
Because of the dually wheels, the interior wheel wells are - well - “doubly” as well. 15" x 36"!
This weekend I start cleaning it up…it’s been sitting and is a little grimy - but nothing awful. I think I have some time and money-saving things going for me here and all in all - I’m very happy with this guy.