Why is my ford holiday not able to turn over?

We bought a 1979 Ford Holiday and we have had some trouble with the charging system. It died a couple of times on our way home. We put a new battery in and it got us home. After the winter.same problem, dead battery. We got another new one and brought it in to see a mechanic, they put a new alternator in and it seemed to work fine
We just took it on a 2.5hour 1 way trip and back and we had absolutely no issues at all.
3 Days later we go to turn it on and nothing… The lights turn on thats it

I don't understand why it could have worked fine on our trip and now be dead.

Any help would be great
I'm desperate and devastated.

Because chevy. Just kidding I'm pretty sure you can take it to the dealership
And they should help you figure out what's wrong with it