Reheating brisket is a challenge - even with access to resources to reheat.
The best option is to cook to 90% finished then crash chill intact in a cooler of ice.
However the reheat over a grill will not be a quick fashion - risking drying out.
Possibly re-inject with choice of fluids for next re-heat (or a pan of fluids as a braise).
Or a possible plan B - Do you have access (or bring) a turkey fryer?
Pre-slice and bag with plenty of au jus (or add low sodium beef broth)
Note - leave bag open standing up during chill to vent properly before vacu-sealing.
Boil in a bag is the best option for moisture retention.
Custom Klose Backyard Chef... of Snot, and a 22.5 WSM
Lang 84 Deluxe kitchen, Lang 60 Mobile - The Damsel II, Lang 48 Patio - The Damsel, Bubba Keg Grill - RIP, Double Barrel Smoker and a BSKD