The heating system is a fairly simple thing. Water circulates in engine and gets warm. Thermostat opens at temperature and warm water moves into radiator as well as heater hoses into heater core. Blower motor blows over the heater core to radiate heat into cab duct work to heat cab.

Thermostat working ok? Is there one? (Known to be taken out in warmer climates)

Heater core good? Even though you flushed the system.. you may still have a problem with something blocking the tubes or maybe the fins could be damaged preventing air flow?...

I did have a problem with heat not working great at one point. My water pump ended up being bad.. It leaked though. Replaced it and all was well.

Also... are there valves to your heater core? I know on big diesels that tends to be an option to shut off the heat so I am mentioning it.