If my gold ring has a stamp inside it saying 18K with an A inside a circle
Why would I be told from a jeweler that it wasn't real gold?
Should I just take his word for it or get a second opinion? Don't want to drive all over town

Let your fingers do the walking. Call several jewelers and ask.

You could ask a jeweller to do an acid rest for you and that would determine what exactly your ring is made of. Usually a letter in a circle would mean the manufacture of year, you should have other marks in the ring which would give more detail, like the hallmark, the assay office origin, etc.