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...
The question is "how". I've gotten lumps in the back of my throat before and the doctors called it "cobblestoneing" it comes from irritation and some people are just more prone than others.
Lots of reasons: they last longer (some say as long as 100 years!). The snow slides off from them much more easily than a shingle roof. They look nicer, and increase resale value.