Interesting. I made the connection in the under hood fuse block you mention. Confirmed voltage at the trucks 7way, but never checked that current was actually reaching the battery, nor have I looked into the trailer connection to ensure there are contacts.
That said, after 2 seasons with our TT I have no reason to think that current isn't reaching the battery. IMO you have a bad connector on the TT and should replace it.
2007 Chevy Duramax LMM/Allison
2012 Jay Flight 29QBH
2012 Jay Flight 26BH (Sold)