Given the immense popularity and influence of his work, did he ever enjoy any financial success during his life, or was he poor like so many other authors and artists? Also how solidified were the roles of the characters in his novel, like were they always perceived the way they were or were they have significant oral history beforehand? Was Cao Cao always considered the bad guy? Was Zhuge Liang always considered a semi-deity like strategist? Was Guan Yu always the guardian god? Did any important historical figures get left out of the RotTK, or did their roles ever become diminished in comparison to their historical importance? For example were any minor characters a bigger deal than they were allotted in the book? Were the heirs of the three main kingdoms really that terrible to be destroyed by the Sima clan? What are the actual circumstances of Liu Bei, Guanyu, and Zhang Fei's meeting? Did they actually swear an oath under a peach tree to live and die as one, just because they met each other in an inn one day, had a good conversation, and decided they had a lot in common, thus they would go out and save the Han dynasty? Sounds super far fetched to me.
That's a lot of questions. I'll answer as much as I can. According the the Dictionary of Ming Biography, the answer to your first question is "not much". I quote, "[Luo Guanzhong]received little attention both during his lifetime and after his death; he is barely mentioned in Ming literature." The sources on him are contradictory and generally unreliable, with one source generally considered reliable by modern historians being a recollection of Luo Guanzhong's friend, who admitted he hasn't seen Luo in sixty years. A source written 150 years after Luo's death says he participated in the rebellion against the Mongol Yuan dynasty, but retired after his leader was defeated and put his revolutionary fervour into writing instead.
While we don't know much about the man himself, we can generally outline how his most famous work came to be. The Three Kingdoms period was already a source of many folk lore traditions by Luo Guanzhong's time, with many Yuan-era folk opera (zaju) based their plot on the period, with dramatic embellishments. What Luo did was to collect these folk opera episodes and connect them all with the written historical record, while adding his own flair. So a lot of roles in the novel are reflective of the popular trends and folk traditions of Luo's time. This is why Guan Yu and Zhuge Liang were especially "godlike", since they were actually deified for hundreds of years already.
Up until about 200 years before Luo Guanzhong's time, Cao Cao's state of Wei was actually considered the legitimate successor to the Han dynasty. Cao Cao was considered the good guy while Sun Quan and Liu Bei were mere rebels. This changed during the Southern Song dynasty, when the Han Chinese got ousted from the northern heartlands and chased south by the Jurchens. The Southern Song began to see Liu Bei's state of Shu as the legitimate successor, since they were in a similar situation with Liu Bei, both wanting to regain the capital region to the north in the name of a dynasty in decline. Since then, Shu increasingly became the "protagonist" in both historiography and folk traditions.
As a work of historical fiction, Luo Guanzhong had to weave complicated historical events into a dramatic and smooth-flowing narrative. So yes, some characters got left out (eg. Liu Chong, the warlord who was assassinated by Yuan Shu) and some roles were greatly changed (Ma Teng was originally a rebel against the Han, in the novel he is a Han loyalist); on the other hand, some characters were embellished (Guan Yu and Zhuge Liang especially), and some were even invented (Zhou Cang, Diaochan). So yes, read the novel with caution and don't take everything it says unquestioned.