Well, to be honest with you, I am doing more research on these "racist newsletters". And when you say - "how he deems some Americans don't look American according to him, then there's nothing that will convince you that Ron Paul is a racist" - you have to understand that "American" is not a race, but a nationality that encompasses MANY races. And I believe that he is speaking from the point of view of a TSA agent and what they are trained to look for.
The answer to your questions is right here in your post.
You say yourself that 'American' is a nationality that encompasses many races.
First of all, why say 'many' and not 'all'? Are there some races that you feel are not qualified to become American citizens? If you answer yes, then I would argue that your viewpoint is itself racist, and if that is the case then of course you would never see RP's comments as racist as they would just be 'normal' to you.
However, if you believe that 'American' is a nationality that encompasses
all races, then the statement that 'someone does not look like an American' is inherently racist. When referring to 'looks,' one can be referring either to the person's physical characteristics or the 'look' of their clothing. There are no other possibilities that could qualify as 'looks.' Since TSA agents all wear uniforms, we can rule out the possibility that he was referring to their attire. Hence (yeah, hence!), he was referring to the physical characteristics of these people.
Physical characteristics can include hairstyle, makeup, tattoos, skin color and facial or body structure. Can you name a hairstyle, makeup style, tattoo, facial structure or body structure that is not 'American'? Of course not, that idea is absurd. Some styles or structures may come from or be influenced by other cultures, but every single one of them can be found on true American citizens. Anyone arguing otherwise would look ignorant, and I don't say that to try to stifle a good debate. Anyone that cares to try to argue this point is welcome to.
So we are left with skin color as the final attribute he could possibly be referring to. But if 'American' is supposed to encompass all races, then all skin colors would qualify as 'American' in appearance. So we can conclude that RP either thinks that certain style choices can disqualify a person as an American or certain skin colors can.