My kill-a-watt meter agrees well with a Fluke. I'm good to go for 120 volt and also have a Hall Effect watt meter for monitoring all use in my Class C. 12 volt is not nearly so critical for me so I use a plug into the cigarette lighter that does xx.xx. Close enough to let me know what is what.
The only item I have that is at all hard to measure is a Nuwave 2 induction cooker. It jumps in wattage a LOT. I can't track it with the hall effect unit and the kill-a-watt can't keep up with the changes either. The only time it doesn't "hop" is I have it on the maximum setting.