It's Cockney rhyming slang. If you want to come up with a slang term for a something, you take something that rhymes "yank → septic tank" and then cut it down "septic tank → seppo"
The only actual slur I know of is that North Koreans call Americans "white devils". Which is ironic if you know North Korea's racial demographics compared to America's.
I've mostly heard 'yanks' in English. I don't know if Japanese slang specifically has a word for USian foreigners as most situations don't really care about which country and, if more specificity than 'foreigner' is required, it's usually going to be some adjective/descriptor (loud, rude, skin_color_here, etc. in the case of people being those things).