How to serve fresh bao buns
Bao are very forgiving.  Food truck aside for a moment, as I don't know what kind of equipment you are carrying, bao can be prepared, steamed, cooled, then frozen.  They can be re-steamed directly from the freezer prior to service with no problem.  You can also make them on the day they will be served. You can steam, cool/refrigerate, then re-steam...or time it so that they are in the steamer during service. Depending on your volume, you can just keep a steamer stocked with the right number of buns.  They can hang out there for a while with no problem.