Inerting a Freighter or Cargo airplane
You are right the compartments of a freighter can be inerted by using a gas that has low concentration of oxygen. It can be any gas or a mixture of gases. Engine exhaust would qualify. If engine exhaust is used then the exhaust will require some sort of treatment to ensure that it does not react with the materials of construction of the freighter and also with the cargo. You will not need much of this gas as there is no requirement of any minimum ventilation. However note that you will not be able to carry animal cargo (pigs, chicken, etc.) in an inerted compartment. Also, precautions will be necessary during cargo loading and unloading. Low oxygen content atmosphere can create asphyxiation hazard.