RAST isn't necessarily very accurate. People with very low RAST values can be allergic.
If your skin test also comes back negative (and you MAY want a "fresh" sample alongside the prepared SPT extracts just to be certain), then you may need to consider doing an in-office food challenge. Negative results on skin testing are at least 95% accurate, but that does leave some room for error, if you KWIM. History trumps testing, but in this case, you really, REALLY need the documentation from your phsyician, so the food challenge might well be worth it. Just make absolutely certain that they can manage anaphylaxis.
If that comes out "not allergic" then you'll need to start looking further at other possible triggeres (preservatives, toxins, environmental exposures, etc), or disorders that can mimic anaphylaxis.
One step at a time, though.