Would you say an RV or a RV?

You use "an" before Words that begin with a vowel sound. If you pronounce "RV" as the names of the letters, it sounds like "arvee", which begins with a vowel sound, so you would say "an RV". If you say the Words that RV stands for, "recreational vehicle", that does not start with a vowel sound, so you would say "a recreational vehicle".