Why do Class C's always leak in the front upper bunk area?

The short answer is they don't...

Many Class C builders have finally figured out that rubber roofs, front facing windows and multiple seams in and around the cabover simply do not work. The quality units now come with one piece fiberglass roofs and one piece fiberglass front/rear end caps. These improvements have all but eliminated the dreaded cabover leaks.

As noted above if shopping for a used C look closely for any sign of water intrusion and if you find any run the other way.

:C