The industry wide nominal voltage is 120 volts. That's what most electrical equipment and appliances are designed to run at. It's also what power utility companies deliver to homes. Sometimes you'll see a reference to 110 or 117 volts though. 110 is what you'll often see on motors.
Yes, it does go up and down but it all depends on how a campground system is designed and what the loading is. With no RVs plugged, you may see it up around 125V and on hot days with everyone running AC units and at meal times, it can plummet towards 105 volts and sometimes less. If a campground has sites that are spread apart quite a bit, the taps on the supply transformer may be set to give a little higher than 120 volts so that when the sites are full and everyone has things running, it won't drop as low. In a small cg with sites close together, you're more likely to see voltage staying closer to 120V. This is assuming that the power utility has their high voltage lines within their normal operating range.
Once you get down to 105 volts, that's bad. It's bad for anything with electronics in it and it's especially bad for AC units. In one AC manufacturers owners manual I read recently, it stated that anything less than 105 volts can cause damage. Surge Guard will shut you down at 102 volts and a PI EMS at 104 volts.
A good idea is to have a line voltage monitor plugged in somewhere inside (Camping World has them). Or a permanently installed one like I've done. That way you can see at a glance what's happening. If you see that you're down to 105 volts, you should be powering down you whole RV to protect it. If you have a Surge Guard or EMS, you'll be protected automatically. Personally, I think 102 volts is too low a threshold and I'd be more comfortable with being shut down automatically at 104 volts.
Of course, high voltage is not good either but you won't see that as often as low voltage. The Surge Guard and EMS units will shut you down at 132 volts.
The only way to overcome low (or high) voltage is to use a Hughes or Franks autoformer. These automatically maintain the voltage within a certain range despite what the incoming supply is like. Hughes Autoformers ::
They're heavy, bulky and not inexpensive though.
If you encounter low voltage and have shut down everything, anything with a resistive only element in it like toaster, coffee maker or griddle will still work although the heat output varies as the square of the voltage. You *could* plug a toaster, for example, into the 20A pedestal receptacle except that it's going to take longer to work.