03 E350 7.3L heating issue

[busman989]

Hi all new to the site glad to be here. My issue is my 7.3L 133k not giving much heat unless under a heavy load or running it in a lower gear, when I get to a stop light it starts just blowing cold air. Ive put 2 thermostats in thinking this was the issue the first was from napa the second was a oem thermostat. Replaced the temp adjusting potentiometer checked the 3 air actuated blendors those work and checked the electric temp blendor that works and doesn't have anything binding it works smoothly. So tried a water pump and fan hub and flushed the system and bought the coolant fill funnel set up that's supposed to help with getting the air out of the cooling system started it up still taking a long time to get up to temp. Also did the dye test that shows you if your getting exhaust into the cooling system but that showed it was ok. Any ideas i'm fresh out. Thanks guys