You are correct. Plenty of politicians have believed that elections were stolen from them. A few have even been as evidence-free as the Trump camp's claims (with one ringleader in the "stolen election" camp saying in court recently that no reasonable person would believe her claims, which, interestingly, is inadvertently saying something about the people who were her target audience and who believed her), while some have had legitimate claims.
Quick point of fact: Sherrod Brown wasn't claiming his election was stolen; in fact, his last (and only) election loss was in 1990 and he has never made that claim about it. However, he was making the claim for someone else's election, so in that way you are right about him.
As much as I dislike Al Gore and Hillary Clinton, they did have more actual votes than their opponents, whereas Trump did not (also, I am aware of how our screwy electoral system works, so no need to educate me). And in the case of Hillary's claims of Russian interference, you and everybody already know
this, irrespective of whether or not the Trump campaign knew anything. So it seems that comparing Gore and Clinton to Trump with regard to their election claims is like comparing apples and orangutans.