I briefly read this thread and had to laugh, I did the same trip, but with less miles a few years ago.
Having never owned a sprinter previously, I was amazed at the similarities as a early bus. Cold, damp and the feel of a tin can.
If I had read this thread a few days ago I would have given you the best advice. Pull the cabin filter out. It clogs and feels like there is no heater.
The next word of advice for any sprinter owner is learn Dr. A's Phone number by heart.
Good luck