To be honest, I never heard schlong used in any other way than sexual.
Growing up in the Mid Atlantic (VA)I'd agree with you, but having spent 6 years in Southern New England during the early 70's I have heard the term used as Trump used it. I would conclude that it's a local usage. However, Trump IS from that area, so I would give him a pass, with the warning that he should only use it with local New York audiences.
Actually, it was used back in 2011, in reference to Geraldine Ferraro and the big presidential loss in 1984. The meaning then was “I lost big time”, and it was used in exactly that sense by NPR.
https://www.youtube.com/watch?v=Yurpga7CXDE
Me neither. From Hardcore, Peter Boyle's character (a p/i working for George C. Scott's character) at a pornographic movie filming, "Look at the schlong on that guy!!"