6.5 diesel coolant loss

Check the connector from the water pump/thermostat housing to the heater
hose (metal to swaged flex hose) that leads into the firewall.  If it's
dripping from that "quick connect" onto the intake manifold, it'll evaporate
and not leave a trace on the ground.

The only other thing I could add (which is not likely in your situation
since you don't mention any other problems beyond coolant loss) would be the
head gasket between the rear cylinders.  On early 6.5's that have been used
for heavy hauling or towing (1993-1997 I think) there was burn through
between the rear cylinders caused by excessive heat under load.  This
problem was fixed in '98+ with the use of a higher capacity water pump with
dual thermostats and a crossover.  If this is where your coolant is going
then it's not an easy fix, but if you do have this issue and your truck is
pre-97 then I strongly suggest installing the later style water pump and
plumbing I mentioned above.

Cheers - Jonathan

Thanks for the replies - Its either the head or the gasket. It leaked
down about 1 psi every ten minutes when my mech buddy pressurized the
rad.  There was water getting into the oil too but its harder to tell
on a diesel. Oil still looked black.