yup. buckets full. :)  it's your drain tubes ... they're clogged up.

to get to them, go OUTside the bus, and undo the two screws in the little
air grills (right behind the rearmost side windows). the grill then kinda
pulls up to remove.

now feel around behind the metal front edge of the hole (where the grill
used to be). you should find a rubber tube, with a sort of "whoopee-cushion"
end on it. that end gets stuck together and the water backs up. squeeze the
end to break it free (be prepared for a bunch of water coming out).

to prevent it from happening again, smear a very thin coat of vaseline on
the inside of the tip. and be sure to do BOTH sides of the bus! :)