I don't know. be very careful when working with propane.
if this happened to my freezer, i'd guess it was the thermostat. if safe and accessable, consider turning off
fuel supply. if adjustable, mark where thermost's setting is.
turn thermostat down and up or hot and cold a few times.
set the thrmostat at a few degrees lower than the mark.. restart the heater and see if tank will now provide constant hot water.