I use this one:
Its made by Prime Products and available on Amazon for about $30.
Another option is to buy one of the better surge suppressors that monitor the voltage, such as the Surge Gard units. They will automatically disconnect your rig. Afterall, if you are away from the rig when you experience a brownout, no voltage monitor will help.
Last weekend while camping in 90deg weather, all of the rigs in the park had their AC going, and I was watching my meter. At night, the meter would read 117~120VAC which is more or less normal.
But in the daytime with all of the AC units running more or less at their maximum, it got down to 108VAC, which is around the point I had cause for concern. I did shut down the microwave so as to prevent it from damage due to a possible brownout, and moved the Fridge over to GAS for the same reason.
But it didn't go any lower, so I didn't take any further action.
The definition of a brownout is a 10% drop in voltage. Since the normal voltage can be anywhere between 110~120VAC, this means the danger area starts around 99VAC. Surge suppressors such as the Surge Gards cut out at <102VAC, which is probably a good point to watch for.
I accuracy tested this meter with my Fluke multimeter and they read about the same, so the accuracy is sufficient.
This meter will not do frequency, but other than a generator, the frequency of the US grid should not be too much of a concern. And I believe the Onan generators will shut down if they go too far off frequency. I'd check the manual for sure though.
Monitoring the voltage in my view is the big issue.