I don't think any reasonable person will associate Camping with mainstream Christianity any more than people associated the Branch Davidians or Westboro Baptist with mainstream Christianity.
I'm an atheist, and I certainly don't. That being said, I don't think he's quite as out of the mainstream as Westboro or the Davidians. Those were cases of just a few hundred people at most, and there are certainly many more people who fervently believe in (and wish for) the rapture, to the point that it impacts their life on a regular basis.
I used to have a co-worker that would would almost always try to tie any news event mentioned in conversation to the coming Biblical apocalypse. My guess (and that's all it is) is that there are tens if not hundreds of thousands of people out there like him.
It doesn’t take a full on belief in everything to lump us all together - jokes about “end of the world” tend to taint the entire group of believers