Xantrex 458 Freedom "filtered" modified sine???

Get sine wave for a few $$ more and have no worries.

A modified sine wave inverter in the old days was basically a square wave. There could be any manner of filtering done to the wave to make it "closer" to a pure sine wave. Also, there may be any number of "steps" involved in the waveform. The more steps the closer it will resemble and actual sine wave. The less steps the more it will resemble a square wave.
Do you know anyone with an oscilloscope? They could show you a picture of your wave form output.

Filtered most likely refers to the removal of certain harmonics that constitute a square wave. By filtering low order harmonics will round off the corners of a square wave.

Here is a diagram/graph of what is essentially a sine wave, referred to as the fundamental, with the 3rd, 5th, and 7th harmonics added, turning it into a square wave. By removing the harmonics from a square wave, it leaves behind the fundamental frequency that we know as a sine wave.



BTW, most electronics don't mind a modified sine wave. Reason is; the first thing the voltage hits is.... a power supply! Turning the AC into DC voltages. Mostly it's motors that run on AC. Very little electronics run on AC. Look for yourself; see what the firs thing is that the AC goes into. Usually a transformer and a DC power supply or several for that matter as most electronics utilize more than just one voltage. Just like your cell phone. The power supply changes the 120volt AC into 5 volt DC.

“we are thinking about changing over to a res fridge”

And give up dry camping/boondocking without a bunch of batteries and solar or heavy generator use?