I absolutely do not reheat tritip; it is a temperamental meat that does overcook and toughen when reheated. You would be better off having a specific person to cook at the time, or serving cold, or using a different cut entirely.

Wlliams Sonoma says,
To store the second tri-tip, let it cool to room temperature, then wrap it, unsliced, in aluminum foil. It will keep in the refrigerator for up to 2 days. To reheat, place the wrapped tri-tip in a 300°F oven for 15 minutes.
Grilled Tri-Tip & Vegetables | Williams Sonoma

Or see this site:
eggheadforum.com/discussion/1176190/reheating-tri-tip