Doesn't gold have some unique material properties that will likely always be in demand? It is malleable and ductile, inert/doesn't oxidize, I believe it is resistant to fatigue, is a good conductor, and lots of others. And as a bonus, it is shiny and pretty. I'm not sure how many of those properties really matter in an active-war Mad Max scenario, but gold has been valued by societies throughout history for good reasons.
If the shit really hits the fan, legal tender will only have some use as maybe toilet paper and kindling.