Anyone have any ideas as to how this could be done?

well, hows about this.....

start w/ a sheet cake -- say a 9x13

cut it length wise so can stack on itself to get thickness of bed of truck.

hack a bit (say 2in or so) off one end and place on top to make the cab of the truck.

then add a wondermold cake (the doll dress cake) on its side for the cement tank. try to bake it so it forms a dome and then leave dome on for so it looks rounded like a real truck does.

then detail the thing out w/ fondant parts (hint the smallest flower formers or parts of paper towel rolls trimmed could be used for the chutes or to form gumpaste chutes

add wheels (cookies or donuts) and away you go.

for more servings, perch on top of a sheet cake that looks like part is ground and part is a form concrete has just been poured into -- if you do this -- the message could be scrawled in the wet concrete!

HTH