This is asking two different questions so I'll answer them in turn.
First, yes, you can use "truth be told" in writing. The verb "tell" is not restricted to speech.
Second, the phrase "truth be told" is not about emphasizing the objective truth of your statements, it's about conveying honesty. This is a rhetorical device used before sharing personal opinions which has a connotation of confession. You can absolutely use this in an essay, but it won't have the effect of convincing people the information in your opinions is true, only that your expressed opinions are honestly held and not adopted for the sake of argument. This might be important if you're writing a persuasive essay about your own moral character, but if instead its a persuasive essay, expository essay, or research paper, it wouldn't lend any weight to your statements.
Rhetorical flourishes that emphasize facts include In fact, in reality, or factually. For example:
In reality, nobody is winning the war.
You'll note I left off the "I think" part. As stated above, unless this essay is actually about your personal opinions, it's not a good way to present your arguments. Because an essay is essentially a long-form argument. The entire paper is your opinion. Any sentences not specifically attributed to another author through quotation or citation are assumed by the reader to be your opinion. So phrases like "I think" are not necessary.