Now imagine that the truck that we have considered so far starts to brake. So he finally stopped (so it has more kinetic energy) at the bottom of the slope (so it has more potential energy rather than at this time). And therefore was able to spend the energy it possessed? 

In fact, it has not disappeared. Simply, it was dissipated by the brakes of the trolley, and it increases their temperature. You have already tried to vigorously rub your hand on your sweater, for example. It is a sensation of heat at the surface of your palm, where it rubs precisely. This is exactly the same thing. 

Indeed, when an object is hot, is that molecules, atoms that compose it are agitated. Is the thermal agitation. More an object is hot, its molecules have more kinetic energy, they are agitated. This explains two things: 

Need to spend energy to heat something (consumption of electricity to boil water, for example), since heat is to move molecules (even if it is not because it is not a movement of all). 

That the potential energy that the truck was at the top of the slope had not disappeared. She had just turned into thermal agitation. 

Temperature, thermal agitation is also a type of energy. It does not differ so much, finally, the kinetic energy. But here is the disorderly agitation of small particles (atoms, etc. ...). So we talk more kinetic energy for large objects. It is said that the thermal agitation is the internal energy.

It is said that the trolley gobal energy, potential and kinetic was dissipated. And as we shall see, the problem it is not that energy is gone, it still drives the atoms, but we can not so recover. The trolley will not recover spontaneously moving. In our view, energy is lost, wasted. This is the second law of thermodynamics.