It moves in a circle. It never ceases moving in a circle. Therefore, at all times, there is a centripetal force.Nothing. The force of gravity will 'keep the truck moving' in a parabola
Even if an object moves in a parabola it has, at each instant, a centre of arc, so there is a centripetal force.
The centripetal force is the component normal to the velocity of the resultant of applied forces.
The only applied force at the apex is gravity. Ergo, gravity is providing the centripetal force.