where the hell does Power Rangers RPM fit in the rangerverse anyway?!
That's a good question. It could go sometime before SPD, if you're willing to believe Earth could be repopulated in only a decade or so (which is no more farfetched than most of the other things you have to accept in the Rangerverse). Perhaps, when Venjix took over, most of the Earth's population was evacuated with help from aliens such as the Eltare, Aquitians, KO-35 inhabitants, Mirinoi, etc. And when Venjix was defeated, they would've returned, along with many aliens they met out there, and thereby set up the SPD future.
Although with the series returning with Samurai
and hopefully beyond, that'll be hard to reconcile. Maybe it's more logical to conclude that RPM takes place shortly after SPD, and all the aliens living on Earth left or were killed off when Venjix took over.