Why doesn't this work on a Mega2560 although it works on an Arduino Uno
You are trying to do an Pin Change Interrupt on Port F (A0-A7). On the arduino Uno, this port does support PCINT but not on the Arduino Mega.

An option is to use the Port K for the PCINT. Instead of A0 and A1 you then use A8 and A9. A few little changes in the code should do the job:

void setup_rcTiming() {
rcOld = 0;
pinMode(A8, INPUT);  // pin 14, A0, PC0, for pin-change interrupt
pinMode(A9, INPUT);  // pin 15, A1, PC1, for pin-change interrupt

PCMSK2 |= 0x0F;       // Four-bit mask for four channels
PCIFR  |= 0x04;       // clear pin-change interrupts if any
PCICR  |= 0x04;       // enable pin-change interrupts
}


and

ISR(PCINT2_vect) {
byte rcNew = PINK &amp; 15;
...
}