heat is marginal at best, even on mild days.somewhat tolerable at higher speeds, but vent temp drops right away at idle

The heater is only run off of two hoses, in and out. If there is a clogging going on anywhere in that it will greatly reduce the flow of the antifreeze going through the heater and not affect the engine at all. Try flushing the system with your heater on high and see if it loosens it up. Heaters that aren't used enough can develop a lot of sludge inside.

Ok not here is information you can use.
#1 check the theromostate, If it is stuck open , replace it. Put in a higher temp, If it is a 165, put in a 185, This will help the heat.
#2 Have the system back flushed, Check the heater controll valve. Some of these are electric and some are vacuum. Is it opening when you slide the controll over to heat, it might not be opening the valve and alowing the hot water into your heater core.
#3 Is low on collent and or water.
Before you start the truck in the morning, remove the rad cap and check the level. DANGER-DANGER-DANGER, DO NOT OPEN THIS CAP IF THE TRUCK IS HOT.
You could be burned verry bad.
Hope this helps