This I highly doubt, or if so this was a truck that was not detailed yet or was like another poster stated an "as is" truck.  Before I lost my job I was in the market for a truck, and after seeing the infomercial on TV,  I took a visit to Gibson Truck World.  The salesman started off by giving me a tour of the facility starting with the full service center and ending in their detail department where they have 2 lifts dedicated solely to just cleaning and dressing the underneath of their trucks if needed.  I saw bills where they spend a lot of money fixing the trucks to make them like new.  Why would they skip something as mundane as washing the underneath?  Does it really seem feasible that someone would put new carpet over mud? come on it takes just as long to do something right as it does to do it wrong.  The dealership has trucks on lifts on his front row that you can walk right under and see everything!  Not trying to start anything but I know Gibson has a very open service center.  Go spend a little time walking around there lot and walk through their service center; I guarantee you that you will see trucks being totally refurbished and you will probably see someone pressure washing the underneath of a truck AFTER its been fully serviced!