There is no modern hortizontal shock application except Airstream. The valving in traditional (vertical) shocks uses gravity for the oil in the shock to move into different parts of that shock under compression and extension.
to later versions of Airstream trailers (not sur eif this design was carried out past 1990) used horizontal shocks to save interior space. The shock valving had to be internally changed because the fluid sits differently in that type of shock and a vertical designed shock will not work.
NAPA doesn't see that type of application either. Or if they do then you need to ask based on year make and model. If they have aplication charts for a '75 Airstream Argosy you will be good. And they don't have a cateloge for that application. Unfortunately this is a dealer only item.
I have a '66 so i don't know if there were changes made for different models and years of Airstream trailers. My bullet has vertical shocks and I can go to NAPA or other places for replacement shocks.
Good luck and educate the NAPA counter person. His (or her) experience could be expanded.