You have a pressure washer right? Of course you do. Why not use it?
Align all of the pumpkins so the dirty side is facing you and spray man, spray. If you do it right, you can sit in a chair and drink a beer while cleaning. If only cleaning the bathroom was this easy.
What's that, you don't have a power washer? Oh. Hmmm. Do you have 5 or 6 quarters? If you do, why not stop at the coin-op car wash. You put a few quarters in the machine and you can use the foamy brush or the pressure washer. Heck, you can even give your pumpkins a coating of "hot wax".
Note: The "hot wax" treatment actually makes the pumpkins really slippery and hard to carry.