A tip is not needed or expected. HAving said that, any group of people sweating in this heat will definitely appreciate cold drinks now and then. If the volume of stuff is massive, a couple of times offering them cold drinks in individual containers they can take and drink by the truck will likely go over very well.
Mike
Quite right, and you might include some snacks too. I often buy indivdual 500ml bottles of tea and soft drinks and let them choose (what Mike has stated is also applicable to other workers that come and spend some time at your home).