I want to comment on your affirmation that formerly most writers were employees under kings. You probably had in mind France in seventeenth century, which had a very peculiar situation. I personally study eighteenth-century English literature, and in this period a great many important writers were against the crown. You will find a plethora of different political biases in perusing those writers.
It is nonetheless true that such writers can only express their own view of the world, but we don't need to limit ourselves to a sole writer. You can grasp something of the big picture by reading many writers. And it is actually done: in the eighteenth century social history was not a common form of writing, and modern historians, when they need a closer view of human interactions at that time, have recourse to novels and other literary forms. They read Addison, and Fielding, and Godwin. Literature is indeed one of the few portraits that remain from those times. You are right in saying that the results of such research may be unfair; but this doesn't mean that literature must be completely laid aside as a source of study. Unless you prefer to leave history aside, too, as a worthless activity.
In short, I think you have reasonable doubts, but they shouldn't induce you to take a radical view of the subject. In all fields of scholarship, it is easy and sometimes profitable to criticize methods, but it is very difficult to suggest alternatives.
I hope I have not been disagreeable.