Originally Posted by markdoane
Does it look like a regular tube P-trap, or is it a cast iron P-trap like the picture below?
If it's like mine ('59) you remove it by unscrewing the drain trim from above.
Right-- if it's original Airstream equipment, you need to remove it from above, while holding the nut still from below with an open-end wrench or crescent wrench or even a pair of large-jaw pliers or channel locks, whatever you can get on there.
From above, the drain has a "+" shape in the middle of it, and you can turn it out by using the special wrench that is made for such things:
BrassCraft Tub Drain Removal Tool - T170 at The Home Depot
Or, if you don't want to go to the hardware store or plumbing supply store, you can try using two screwdrivers crossed to get it out.
Depending on PO fixes over the years, there might be some kind of sealant between the drain and the pan. Putty, silicone caulk, etc. You should remove all of that with a razor knife (take care to protect your pan if you plan to reuse it) so that the drain can spin freely. Even then, decades of corrosion might make it difficult to remove. Hit it repeatedly with a heat gun and it should eventually break free (again, be careful not to overheat your pan if you plan to save it, try to keep the heat on the metal).