For the rear fender skirt, try
RV Salvage and Surplus Yards by State.That is an old list, so some might not be in business. The 2 largest are Visone in KY and Colaws salvage yard in MO.
As to the air springs, some brands have a sticker on the metal end cap, trouble is you must remove the air spring to view the sticker.
I"ve read that some have the make/model molded into the rubber, but have never seen one.
Just for accuracy purposes when posting, an air bag fits inside a coil spring and assists the metal spring. What we have is air springs.
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2000 Winnebago Ultimate Freedom USQ40JD
, ISC 8.3 Cummins 350, Spartan MM Chassis. USA IN 1SG 11B5MX,Infantry retired;GS Life member,FMCA
" My fellow Americans, ask not what your country can do for you, ask what you can do for your country. John F. Kennedy