In the pre-'07 Sprinters, exhaust smell in the truck could be associated with a leak of exhaust around an injector where it seals to the head. Don't know about the V6, but with the I5, it's easy to check for that condition by removing the plastic engine cover and checking each injector well for oily or tar-like deposits.

I have a 2004 FL SHC 2500 and my exhaust is under the middle of the vehicle, like yours. When I open the back after stopping I smell the fumes. There is supposed to be a law in the US that a tailpipe has to exit the body of the vehicle, but, after at least 10 calls to government agencies, no one knew what I was talking about. I gave up trying to find out. Do you leave it running while you sleep? They do make an extension exhaust pipe the comes out the back. That is what I plan on doing. But I will also check out the leak that guy was talking about. The dealer said there was nothing wrong with my exhaust. I get fumes in the cab, too. Good luck.

Thanks Talkinghorse43.
Just had mine a 2005 2500 sprinter fixed after 6 months of fumes in the cab.
Drove me crazy, #2 injector was filled with tar, the cover smelled of the same exhaust like stench, I knew right away the smell this was the problem. Seal ring was oval and clearly leaking.
Engine runs quieter now also.