How do you answer when a stranger asks about MPG?

I don't try to be cutesy or sarcastic, I simply tell the absolute truth:
"Damifino, I don't check it or worry about it!"

However, I do carry a logbook, and enter all fuel purchases in it, so if I ever really wonder about my lack of fuel economy, I can figure it out. IMO, those who are really concerned about fuel economy should not have an RV of any kind!