Going by my own experience , those who are "new in the truth" probably do believe it all - I know that did once, anyway.
However, the ones that manage to remain with the JWs would have to develop some sort of a coping mechanism in order to stick with:
eg. in public, giving lip service to WTS theology; while privately turning a blind eye to the more extreme requirements of their religion.
These would tend to be ones initially attracted to the JWs by reasons other than doctrinal purity (and there are those around who were first drawn to the dubs by their apparent purity of doctrine - I know, I was one!) . Such persons would likely rationalize the whole thing along the lines of "Whether right or wrong, Jehovah's Organization is a safe place to be in", or "Better to be wrong and in Jehovah's Organization, than right and outside it", and so on and so forth.
So, in answer to Minimus's question, most of those who are "many years in The Truth" probably don't believe it - not the more extreme parts of their religion, anyway (which, let's face it, actually constitutes the most of it!)
Bill.