I think the best example of slavery under feudalism can be found in relatively modern times. The Russian empire under the Romanovs were the last absolute monarchs in Europe who still actively tied the peasantry to the land.
In Russia I think the best example of how powerless the serfs really were is in the drafting process for the military. The contracts could be 30-40 years, which was basically a lifetime. Conditions in the military were so bad their families would throw their sons funerals before they left.