Many people today still choose not to believe it happened:
Well there is an active genocide where an entire country is being literally bulldozed into rubble with all survivors shot by snipers, heavily documented with endless videos, and the worlds most powerful nation is united in supporting it, so…
Even literally during WW2, the US created concentration camps on its own soil for millions of American-born citizens who happened to be Asian. These people had their homes, businesses, and property just brazenly stolen by their white neighbors while they sat in camps in the desert. They had nothing to go back to. This is why many West Coast small towns have “Japantown” neighborhoods with 0 Japanese folks.
So yes, I’d say it’s safe to say people didn’t “believe” the WW2 camps.
I imagine most people kind of knew what happened in those camps but there was just enough distance between them and the actual suffering that they could ignore it. See the people calling for less immigration or sometimes actual deportation; those immigrants will suffer for it but the people calling for it will not see that suffering. See people eating animals; most could never put a live chicken in a blender or a pig in a gas chamber but since they are just far enough away from the act they don’t mind supporting it.