71 gallon water heater doesn't maintain hot water
This happened to me when the water heater in a rental house was replaced by the landlord's "handyman".   Check that the cold inlet and hot outlet pipes on the heater weren't installed backwards.

The inlet should have a tube so that the cold water enters at the bottom of the heater.  The outlet should pull from the top.  That way the cold water pushes most of the 71 gallons of hot water out of the heater.  If you hook it up backwards, the cold and hot mixes immediately, and quickly becomes lukewarm.