The main thing that determines the expected future life of ANY coach is how it was maintained by the previous owner(s).
If the coach you are looking at has complete service records going back to it's first owner, that is a big plus.
If it does not have those records, then you are taking a big chance that it was maintained properly. If you only have the owner's word, or salesman's word that the oil was changed on a regular basis, then you are taking a chance that a person with a lot to gain is telling you the truth.
If the owner says that he did all of the factory required service on his coach, you have to determine whether or not you want to trust this total stranger as to when and how he performed that service. Not all people are competent mechanics and are capable of doing ALL of the factory required maintenance on a diesel coach.
Look for coaches that have complete service records and also complete manuals of all systems.