You should really tell them that tri-tip is a premium meat. It is one of the priciest meats to vend actually. That being said...
Smoke it to 125F, it will finish rare. Once you put it into the chafers, it will raise in heat to a servable temperature. I have found very few people have high expectations for tri-tip. Oddly, they will eat 145F tri-tip and be very happy.
Keep the meat whole until you slice, keep it wrapped and coolered, it will not overcook while resting. Slice just before loading the chafer, as it dries out crazy fast once sliced.
me: I don't drink anymore
Yelonutz: me either, but, then again, I don't drink any less