Coolant leak, Heater not warm

[cj-10]

I have a coolant leak that happens only after the car sits awhile. I cannot locate where it is coming from after removing the plastic shield and inspecting the engine from the bottom. It is not around the radiator nor the secondary water pump. I already did the valley pan and collapsible tube replacement. In March. Also the heater is no longer blowing hot air. I am starting to think there is an issue with the thermostat or water pump. I lose about a half gallon of coolant every 10 days

I have a 2004 4.4