Problem with Heat blowing luke warm

Hello all I am new to , also new to taking care of cars and fixing them too. Need a little help with my heater problem. To start I have a 1989 Cherokee 4.0 AMC 242 engine with 4 wheel drive with about 200,000 miles. Now to the problem my heater is acting very weird pumping out luke warm air from the vents and cold air at my feet. The heat out of the vents is also warmer on the passenger side than the driver side?. The blower is working fine (at least I assume so since it's blowing). Recently changed thermostat, flushed the heater core. Any other ideas or answers would be greatly appreciated. One more thing if you do have an ideas that could fix my problem could you please go into detail seeing as I have probably never done or seen it done before.
-Thanks

Under the hood, where the heater hoses go through the firewall in to the airbox, there is a coolant control valve with a vacuum line attached to it. When you move the "heat" slider on the HVAC control vacuum moves the control valve to allow hot engine coolant to flow into and through the heater core.

Check to see if the little arm on the other side of the vacuum motor moves the valve when you adjust the "heat" control. If it doesn't, test the vacuum supply, and if that is Ok you need to test the valve itself.

Heater control valve is absolutely suspect. If it is old, just replace it as they were under-designed from the factory. When the plastic gets old, it has a tendency to disintegrate, leaving you with a massive coolant leak and a possible overheat. They are about $30.

Did you get enough fluid back into the system when you changed the thermostat?