Interestingly, all the words you mention have very different origins.
Plátano, which is also the word used in Spain, clearly comes directly from latin platanus. There is the possibility that this had its origin, ultimately, in greek πλατύς ("broad", as a reference to the leaves), but as far as I know it is unclear.
Banana (or banano) entered Spanish/Portuguese through Wolof.
Guineo quite probably comes from the toponym itself, the country in África, but I cannot be sure. I actually had no idea they used this word in El Salvador.
Cambur is quite interesting! It comes from Guanche, a language native to the Canarian Islands that must have died out at some point during the XVIII century, due to the colonial activities of the Spanish empire. Not many words are preserved from Guanche and much is unknown about it, so that makes Cambur more interesting. Why did it arrive to Venezuela and never to Spain (where it is an unknown word except for specialists) is unknown to me. I would be interested to know.