Your Whirlpool dryer drum is not porcelain. What kind of discoloration? It's common that they pick up a blue tinge if jeans loads are run a lot, but that's not to be considered damage nor does it bleed off onto other clothes. Keep in mind that a maladjusted gas burner can contribute to discoloration, as any combustion byproducts are passed through the dryer drum (and clothes) with the heated airflow.

PLASTIC insulates much better than metal. Hold a stainless steel bowl or pan in the palm of your hand and pour boiling water into it ... see how long before you drop the pan. Then do the same thing with a plastic bowl (that's sturdy enough to take boiling water) and see how much longer you can hold it. The metal pan gets hot because it's letting the heat OUT, not holding it in.