Heater blows hot for about fifteen to thirty seconds when first turned on and then goes lukewarm to cold

Check this. First, make sure the coolant is full. Next, with the engine running, hot, and the heater on, feel both heater core hoses. Both should be hot. LEt me know what you find.

Joe

Flush the cooling system without a thermostat, especially the heater core, then install a new thermostat and coolant. If the heater core will not flush easily, it could be blocked -- in which case a new heater core should be installed.

The is an air door actuator that has gone bad. this guide can help fix it.

https://www.2carpros.com/articles/replace-blend-door-motor

Please run down this guide and report back.

Try pulling HVAC fuse inside truck for sixty seconds with key on it may reset actuator. It would be the blend door actuator if heater hoses are hot when engine warm.