2007 Dodge Nitro 4.0 RT Lose heat when driving

ok so im at a loss at this point, I purchased this 2007 dodge nitro in march of
2020, Since then i have tried 3 thermostats and a heater core to keep heat in
the vehicle. When at an idle the heat can roast you out of the vehicle. When
doing 55 miles per hour you can see the temp gauge dropping and on the
highway the temp gauge sits just below 1/4 of the temp gauge instead of
around half when the suv is at an idle, I have plenty of fluid i have burped the
system, i have replaced the thermostat and refuse to believe i have gotten 3
thermostats stuck open. What else can i do