Davco filter (clear) housing empty overnight. No FUEL !

Yes, once you broke seal, but not when connected and pulling it up from suction tube in tanks; would just stay neutral when key is off.
I would guess you have a bad valve on your fuel return and a leaking check on your devco.
My truck fuel returned to the top of both tanks, so with any component weak it could have easily siphon out fuel.

 

By your description you could also have an inlet restriction from filter to the tanks. Blockage not allowing enough fuel flow to the Davco. Does it run ok and maintain height in the bowl when running? Backyard check.... put the line to the davco in a bucket. Put a small amount of shop air in each tank and make sure you have a steady unrestricted stream of fuel coming out the line. If not trace it back to the source.

Also make sure the tank vents are clear. If not it can pull vacuum on the tanks and cause the same issue.

 

Ya can't tell you what you have on a truck with a Detroit, I have a Cummins, would be easy to tell where return valve is and where.
Now I have ran lots of 60 serires in shrimp boats, but #### that was 15 plus years ago.

 

so what was the issue and how did you resolved it

 

I just went through this, it was a plastic ball valve in the davco filter housing it had some trash and the spring that hold pressure on the ball was getting weak the part was around 20.00 at freightliner.