The Rotbots come from a future where the Clean Team won.
Considering the future that the Rotbots live in is considered "rank hygiene" to the Grosseries there, it makes sense that they come from a location where the Clean Team, the antagonists that drove out the filth in their failed present attempt, would have been successful in another timeline. The cyborg attachments are probably the only thing keeping the Rotbots alive, giving them good reason to want to go to a timeline with proper filth. The Grossery Gang is in the same universe as Flush Force.
Even though they are both by two separate toy companies, Grosseries are still seen in the background of one of the commercials. While this simply could be prop reusing for set decoration, the fact it's specifically the characters, paired with how vast the Grosseries' side of the world is, makes it seem like Flushville isn't too far from Cheap Town.