Hey, I'm all for proper calibration, and I'll use a Fluke before I use the Harbor Freight meter (though I have a HF meter that I keep in the RV)...but I'm not all that concerned about 2-3 volt difference....in the day to day stuff.
Yes, low voltage would be an issue for the A/C or some other components in the RV, but I feel that a quick check with the meter is good enough. Now, if the meter is showing a difference in excess of 10VAC at a campsite, I'll be worried.
As someone mentioned, it's really when you are working on tight clearances and low voltages that these numbers matter.
The the average RVer, any meter is better than no meter! And as with most things, try and get the best quality you can afford....it will be worth it in the end:)