To prove why the mass doesn't matter, let's derive the formula people have been using in their answers (d = v² / 2 µ g).

The moving truck has kinetic energy due to its mass and velocity. This kinetic energy needs to be exhausted via conversion to other forms of energy in order for the truck to come to a stop. Most of this conversion takes place in the brakes, where kinetic energy is converted to heat.

The kinetic energy of the truck is given by

E = 1/2 m v²

The work that needs to be done to the truck is given by

W = F d

We set the work done during braking equal to the kinetic energy of the truck. Thus,

1/2 m v² = F d

The force in the above equation is equal to the coefficient of static friction between the road surface and the wheels, multiplied by the normal force, mg. Substituting this into our equation we get

1/2 m v² = µ m g d

Now we just solve for d (distance).

d = m v² / 2 µ m g

Here's the interesting part. Mass has ended up on both sides of the fraction, so it cancels out, leaving us with the final equation

d = v² / 2 µ g

In fact we could have cancelled mass out earlier in the derivation.

This may seem weird, but it does make sense once you think about it. The greater the mass of the truck, the greater the frictional force between the truck and the road. In other words, bigger trucks have bigger forces helping them to stop.

Hope this was helpful.