Strangness with a Kill A Watt

Question....
is my Kill A Watt meter going bad... or is it not?

So.. I have a Kill a Watt.. and it has seemed to work OK...

Recently I was boon docking... I do not have an inverter in the RV, so I hooked my portable one up to the coach battery... and then ran a cord inside.
It is the first time I have camped with a CPAP and wanted to run it...
Plugged in the Kill A Watt to the power cord and then plugged in the CPAP....
wow... voltage went from 120 to about 138 or so.... Yikes!!

unplugged the CPAP... tried again.. same thing...
weird..

pulled out my digital volt meter...
voltage at the cord (from the inverter) was reading 120v...
plugged in the CPAP.. and carefully measured the voltage.....
120V... no spike like on the Kill A watt.....

So.. is my kill a watt dying... or.. another thought I had is the inverter is NOT a pure sine wave.

It is a 1000w Black and Decker that has worked for quite some time...

Thoughts?

Exeter