Dirk Miller chats with a customer outside Miller’s Smokehouse in downtown Belton. (Photo ©Chris Wilkins) One of the best-kept secrets in Texas BBQ is located just a stone’s throw from Interstate 35 in downtown Belton. The next time you’re speeding down the freeway on your way to stand in line for two hours to get BBQ in Austin, you might want to take the Main Street exit in Belton to 208 N Penelope St., the home of Miller’s Smokehouse. I dropped in for a late lunch at Miller’s on my way to Austin on Friday. This was my third visit…