Measure AC Voltage with Arduino doesn't work?

Other Hardware General Electronics
20 
void loop() {
 unsigned long ts = millis();
 float V;
 float minV = 4;
 float maxV = 0;

 for (long i = 0; i <= 100; i++){
  V = ads.readADC_SingleEnded(2) * 0.000125;
  if (V > maxV){
    maxV = V;
  }else if (V < minV){
    minV = V;
  }
 }
  Serial.println((maxV - minV)* 110);
}

For starters, your declarations should not be in the main loop, and for seconds, you should put your
samples in an array inside a loop with nothing else in the loop so you get the fastest sampling speed,
and for thirds, you can use loops later to process the collected data.

Look up how to initialize an empty array
Look up how to append an array in a loop
Look up how to find min and max values in a loop
and last but not least , reread this:

If you know that RMS stands for root-mean-square, you should be able to figure
out that you calculate the mean of the squares of the samples, then take the
square root.

In particular you need to remove the DC offset first, ie:

  1. calculate the mean
  2. subtract the mean for every sample <<< correction: from every sample
  3. square every sample
  4. calculate the mean value of these squares
  5. take the square-root of this value.

also, there are plenty of examples on the web of how to do fast analog reads with an arduino.
What should be obvious is that you should not be doing anything else in between reads because
it reduces your sampling time.

Here's a freebee:
analogReadFast Library