I have never been a slave, but even as a submissive, my former Dominant wanted to know what was in my head. He didn't make me write it though, he simply engaged me in conversation and personally asked what I thought about different things. We had a discussion on it, and I was allowed to have my own opinions, but his word was always final.
To the question if you have the right to an opinion, the way I was taught about Master/slave is that yes you can have an opinion, but that is a position. It's how you feel. Your opinion however, cannot interfere with you carrying out your Master's demands. You should put forth the necessary effort to complete his request, and maybe in time HE will see that the points you made in your argument are true, and you really don't have time to write such a complete journal. If you are not willing to follow this one seemingly harmless command, are you really willing to be his 'slave'? According to my studies, you have to follow his demands/commands/requests no matter what because being a real Master, they are what you need to be a better person. You are not to question his commands, but you do not have to like them.
Again, I'm not slave. I could never be a slave. It's a very serious position, and I just can't do it, but this is what I have been taught by Dominants and Masters who have had many submissives and slaves in their lifetimes. I wish you the best in your quest for resolution.