Should the underside of the rear fender look like this?

[unflagging]

I was lying under the passenger side of the truck

In front of the rear wheel, looking forward

This is the underside of the bed or rear fender

Look at the black undercoating.

Was this bed replaced?

There is a stark contrast with how clean black it looks.

 

[Mike GMC]

It's possible that Isa new bed side.  Either way, it appears to be in good shape.