Heater - 2000 GMC Sierra Truck

The heater in my 2000 GMC Sierra truck (non Air Conditioned) is not getting very hot. Anti Freeze has never been changed / flushed; but the reservoir is full. Does this sound like a an issue with my coolant and the system needing flushing or a problem with the heater controls. The temperature gauge shows that the engine is heating up normally. When I turn the heater temperature control on the dashboard, it rotates like it has always done in the past - but it does not heat up like it used to - the air coming out fromt he heater only gets slightly warm. I live in Alaska and it is getting cold outside and I need a good heater.

Thank you

Your thermostat could be sticking open.
It’s overdue to be replaced, IMHO.
I change mine every 4 years.
So change your coolant and thermostat.
Might fix the problem and needs to be done anyway…
This will also save you gas.