First off the HD requirements for reheating cooked foods are 165F internal temp. So at any less temp you are techincally taking a chance.
With that out fo the way Tri-tip IMHO is best served rare. That means any safe reheating will cook it from rare to med well. Now if that's OK then your crock pot method is just fine. With enough liquid and I'd also add some beef broth you can reheat fine and it will make fine Italian beef sammies.
Oven foil and broth would also work great. I don't think I'd try to put in 120F water for an extended period.
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