How do you keep the coolant at the proper level in these newfangled trucks that have a teflon tank to put coolant in, instead of a radiator?

The plastic tank will have level markings. Usually MAX and MIN. As long as the coolant is in between these marks all will be fine.

Its Not "Teflon", Believe Me. Just polypropylene.

There  is a low and hi mark on it. Cant see them? Maybe 1/2" from top, just like the old fashioned copper radiator  in my old Chevy.

Fill to the "full" mark. Snot rocket science.

You keep the coolant level in the normal area on the newfangled tank.

The tank has two idiot lines.  Max and min or full and low.  Service to max or full.

I can assure you it has a radiator. The tank you are referring to is the overflow tank. As the antifreeze mix heats, it expands. Since the radiator is a pressurized system, the excess volume of mixture has to go somewhere to avoid over-pressurizing the system. The manufacturer provided an overflow tank for this purpose. You fill the overflow tank half full to the mark on the side of the tank. When the system is hot, the tank will be a little more full. When you stop the engine and the system cools, the antifreeze mix shrinks. It takes up less volume. Because the radiator is pressurized, it sucks in a bit of antifreeze from the expansion tank back into the radiator.

This process, when working properly, keeps the radiator and engine coolant passages full of antifreeze. Just keep it at the full mark when cold. 

newfangled trucks, that's funny, you sound like me, just even older.