Not everyone owns a Sprinter chassis model. I suspect it's a part which might have been part of
the captain's chairs which came with the chassis, and weren't added by the van conversion process,
but I may be mistaken.
There are 2 places I'd visit to try to find out on a Saturday.
1) A local Chrysler dealership might have someone who might be able to remember what it's used for,
from when they were Daimler/Chrysler and sold the Sprinter chassis model vans.
2) A Daimler (Mercedes) Benz, or Freightliner dealership, that sells Sprinter model vans.
They might be able to help.
You could also try posting your picture on the Yahoo Roadtrek users group, or in the class B
section of the RVNet forums, but you'd have to join them before you can post stuff, and their
simple vetting process for new members can take a few days.
Best I can do, sorry.
(I see it in figure B-P1A in the 2005-6 Sprinter Roadtrek PDF owner's manual, but there's no mention
that I can see, of what it does. Tried pushing it? If it doesn't go boom, maybe you can figure it