Wei,

It's possible that you have a small leak somewhere. Such leaks may not
leave water under the truck but will leave a white spot where the leak is
occuring. This is really obvious on black radiator hoses. How old are the
hoses. It may be time to replace them.
If you believe that you have a head gasket leak, take your car to a
mechanic. A leaking head gasket could put water into the oil, which should
be pretty obvious since you are losing gallons of fluid ,or the leak could
be putting water into the combustion chamber where it gets evaporated and
comes out the exhaust.
A good mechanic should be able to diagnose your problem.

Dave M.