You are correct. This issue is a giant killer for DT. He is trying to run a cheap campaign but to get this message out he will need to do some paid media.
In the primaries, Trump could get lots of free media by letting something salacious slip during an interview. It could be total crap Jim Hoft or the National Enquirer but Trump would slip it in and say “somebody should look into that” and now the media had license to quote unsourced garbage.
Could he do the same thing to Hillary? Maybe. If the media doesn’t take the bait, bring it up during one of the debates, particularly the seizure issue. If the media won’t run with it, Trump can tweet the media clip and demand she release her health records just like Reagan and Dole.