If you are able to extend your visit the 2nd annual Wild West BBQ Bash takes place Saturday in Santa Maria's Priesker Park; People's Choice tastings are $2, and the competition includes tri-tip alongside the more traditional competition meats.
Are you looking for traditional "Santa Maria Style Barbecue", or just Tri-Tip?
As you probably well know, the best and most authentic way to find Tri-Tip is in the parking lots in santa maria on a weekend, served with salsa, pinquito beans, toasted french bread, and green salad. ( http://santamariavisitor.com/cm/santa...)
Farmers Market in Down Town San Luis Obispo on a Thursday night is your best bet for a traditional Santa Maria style presentation with the sides.
For a lunch i highly recommend the tri-tip sandwich from the Arroyo Grande Meat Company in old town arroyo grande. It is a traditional butcher shop that serves lunch sandwiches using their meats. They serve the tri-tip on a sandwich with bbq sauce and thus is not really traditional santa maria style.
Also, Jockos does not serve Tri-Tip but their steaks are amazing, prepared over oak in an open pit, and with the traditional sides. I recommend the spencer.