I can tell you from experience that roaster ovens are a poor choice for reheating...found that out when I reheated meats for my daughter's wedding a few years ago. They have side heating only...no bottom or top heat...so you have to tend the food quite frequently to avoid dried-out meats (overcooked) on the sides, and under-heated meats in the center and bottom...lot's of turning involved and it takes longer than it should to complete the reheat. On top of that, the ones I used pulled 1,450 watts per roaster...the electrical requirements can become an issue. An oven with multiple covered roasting pans would be a far better choice. If that's not possible consider some cheap 6-qt crock-pots. You can only fit about 15 bone-in breasts each, but they only draw about 400-450 watts. The crocks will heat from all sides and bottom after they get up to temp.
If you add just a small amount of broth to the cooker for steaming will help keep them from drying out. If you can probe them for IT when you grill, try to get them off as close to 165* (min) as possible, then when you reheat take them to 165 again and hold at 135(min)-140*, if they need to be held at temp.
I'd ask around with church members and see if they can bring crock pots to help you out. A couple power strips and 12-14AWG cords should handle 6 crock pots (follow the ratings on the power strips and cords).