LINDA:
Good morning to you in Canada. YES!!!, Linda you certainly can do what you would like... pre-cook your chicken. Cook the chicken to 180 internal degrees on your therm. Then re-heat the chicken on your Grill the next day.
The veggi Brocoli & Potatoes you can steam them each. Then you can re-heat each in a alum. foil tent on your grill.
OR
You can make "BAKED POTATOES" by boiling them in water on your range top it only takes about 25 minutes this can take place before serving your chicken. The potatoes will be much more moist cooked this way & quicker. Place the potatoes in alum foil when serving.
Good luck & have a nice day young lady.

~CHEF DUNASK.