dh bbqs it all the time - matter of fact, he's on his way home to bbq one tonight.
We don't ever use a marinade, only a dry rub of salt, pepper and garlic powder.
Since dh is the Q-er, I'm not the one who knows all about the heat/timing.
Normally I don't send folks to google, but this is one instance where it's needed, I think.
The very best info you can get on BBQ tri tip will be by googling "Santa Maria Tri Tip Barbeque" ( or BBQ)
That's where it originated, and they seriously have the best recipes. Also, not only will you get some good recipes, you'll also get some great info.