First a little clarification....
Title states 'inlet tube' but you say 'inlet valve' in body of post and talk about coolant.
So I assume you have an Atwood WH with motor assist heating with tubing from engine to WH with hot coolant running thru it and you have a pin hole leak in that tubing.
Atwood tank is aluminum and the motor assist tubing at WH is factory installed and welded into tank. So I would assume tubing from engine is also aluminum tubing.
Clean up area around pinhole leak, rough it up with course sandpaper and use JB Marine Weld
