Cruise works once in a while,Gas mileage went from 18 to12.4

A thermostat in a car is not like at home...it does not control temperature...it simply opens when the pressure gets hot enough to open (this lets the coolant go through the whole system). Also, the thermostat has nothing to do with either cruise control or your gas mileage. Sounds ike you have multiple problems which have to be tested by someone with the tools to do it.