It was two words in one sentence, each one of which depended on the word 'were': Americans were fully trustful toward their mass media in those days and [were] absolutely negative about Russia
Fair enough. The answer to your question is that both of your examples are grammatically fine but awkward. English writing style recommends, where possible, using simpler forms and fewer words. If you can replace "were fully trusting toward" with "fully trusted", you probably should.
But then you have the challenge of making this parallel with "were absolutely negative". There are several tricks, but the most useful is simple vocabulary. What do you mean by "absolutely negative"? Is there a single word that means the same thing, but can be formed into a verb or an adjective (depending on the first part of the sentence), and possibly with additional nuance to give your opinion more depth?
Some options: antipathy, animosity, antagonism, enmity, hostility, rancor, opposition.
In the same way, what do you mean by "fully trusting"? Do you mean to imply that Americans were naive or gullible by having this trust in the mass media? Or rather, do you mean to say that the media themselves were more trustworthy in those days?
Some options: credence, confidence, faith, certitude, surety, conviction.
This gives you plenty of interesting combinations:
In those days, Americans had complete confidence in their mass media, and [had] absolute antipathy toward Russia.
In those days, Americans were naively credulous of the mass media, and [were] overtly hostile toward Russia.
Side note: If by "in those days" you mean "before ~1990", then it's probably more accurate to refer to "Russia" as "the Soviet Union" or "the USSR".