I think you're right!

All this time I assumed the clear lines were coolant lines, but I was wrong. They glow in the dark like there's no tomorrow (after I put a UV dye in the coolant), so I think coolant gets mixed with diesel in the heat exchanger, so I'll order it and replace along with GPs.

Thanks! I think I'm glad I didn't try the radiator stop leak thing. Will go with this solution, possibly this coming weekend.

This is the picture that clued me in - found on another forum, showing a black fuel line going into the heat exchanger. The clear line that comes out of it is fuel, and since it's glowing under UV light, it must be getting mixed with coolant when getting heated up. This explains why my coolant level doesn't go down on long trips (engine is hot), but goes down a lot when I have multiple cold starts.
https://www.dropbox.com/s/staorrvz3pebqeq/Mercedes%20Fuel%20Line.jpg