[flyingtim01]

No need to make a mess with flushing & changing the thermostat if it was working properly before and you're not looking to complicate things. Not a bad idea to get a new radiator cap.

Fill overflow tank to full cold mark. Make sure radiator is full. Loosen radiator cap (old or new) and start engine. Turn heater all the way up, blower, on medium.

I'm guessing that once it gets up to operating temp and the t-stat opens, you'll see the level in the overflow tank & radiator drop. Keep them both full while the truck runs.

Once you get a steady reading near the 210 mark on your temp gauge, you can close up the radiator cap and top off the overflow tank if need be. You should also be getting good heat in the cab at this point.

If you still have no heat, and the temperature still climbs past the 210 mark, and you haven't had to add any coolant yet, then it's probably time to change the thermostat (because its stuck closed).

More than likely you're just air bound and the system needs to burp a few times and purge the air. You may even need to bleed it more than once to get the air out of the high spots once and for all. The dead giveaway for air in the system in a temp gauge that is constantly going up and down 20-30 degrees.