Midway had the 5.11 Rush24 backpack available. It's has about the same dimensions as the Swissgear pack I had examined at Sam's — roughly 20 inches high, about 14" wide and, depending out how much stuff you have in it, 8" to 10" deep.
To be ready to pull out, I would slide in my laptop, put a water bottle or my 16-ounce Stanley thermos, or both, in the side pockets, throw a handgun, a Bible and maybe another book or two in the main compartment. That would get me through a couple of days with no problem. I still have room for a spare pair of jeans, a couple more shirts, and maybe a pair of running shoes if I wanted to stay somewhere three or four days.
The bag itself is covered with Molle straps so it would be easy to add on more pouches and equipment. Everything seems solidly attached and neatly sewn. The backpack straps have releases which are helpful in getting the bag actually on my old, stiff shoulders. The grip on top is supposed to be good for 2000 pounds, and that's what's I'll grab first 90% of the time.
This goes right in the front seat of the truck or the floorboard -- front or back without any problem. Being a "water-resistant" backpack, I could wear it when I'm on my bike, or, since it isn't overly large, I think my ROK straps would hold it quite securely on the pillion.
I appreciate the input, and this seems as though it's going to work pretty well. The features and the size are just about perfect for what I wanted -- about the same room for travel gear as the old duffel bag plus laptop space and protection.