Guarantee is a bad word to use.  The Title V inspection is not a guarantee or warranty that the system will function properly in the future under the same or different conditions.  There are simply too many variables that impact a septic system.  If the new family moving in has more people living in the house you would expect them to use more water which taxes the functioning of the system.  Families with children tend to use more water for laundry and personal needs than retired couples and people who have lived in the city may not understand that private septic systems can only deal with a limited flow of water.  The assurance that you have with a Title V inspection is that it is functioning properly now.