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
An American expat in the office did exactly this - he offered no tip but cold beer to the movers. I will do the same, except I'll have a selection of non-alcoholic drinks, as well as beer at the end (wouldn't want them handling works of art in a drunken state!). Thanks!