The Expedition Duffle Bag 140L is large enough to put a couple of weeks worth of clothes in, and still have room for some groceries.  8,540 cu inches of capacity to be exact and this bag is airtight, waterproof, and no dust is going to get on your gear with you throw it in the back of your pickup.  

These waterproof bags are made with rugged 1680D ballistic Nylon combined with military-grade Nylon webbing and TIZIP® water / air-tight zippers.   Each bag contains a water-resistant zippered pocket for valuables inside the main compartment. The buckles consist of anodized die-cut aluminum making them extremely tough. Removable backpack straps allow for easy stowing during air travel for checked-baggage. The oversized zipper construction creates an extremely large opening for easy access to your gear while reducing stress on the zipper, and there is a webbing loop at the end of the zip to attach a safe lock.