I see that some of this has already been said, but I'll weigh in anyway.
1) Convenient water and small bowls for the dogs so you can give them some at every stop, without unpacking the truck and without wasting a bunch of water. 2 liter coke bottles work well, and can be slid in around the kennels;
2) If more than one gun, and one of them has older steel, then all the right shells for each gun. By that I mean you need lead, you need non-tox, and you need non-tox friendly to your older steel barrel (bismuth, Tungsten-Matrix, or something else non-steel and non-Hevishot), or you need to just decide that you'll only shoot the hard non-tox stuff out of the newer gun;
3) Tow strap (I've never needed it, but always glad it's there);
4) A couple of extra bottles of whisky, or something that's a nice gift from wherever you come from (local beer, hams, jams, etc.), or a Wal-Mart gift card or two, for that nice farmer who stops, takes pity on you, and says "come hunt my place";
5) lighter gloves than you think you need, unless it's going to be brutally cold;
6) lots of orange for everyone, including orange gloves in my opinion;
7) plenty of on-the-go snack or lunch food, so you can eat between spots, and plenty of drinks, including just plain old bottled water;
8) extra rolled-up bedding (fleece blanket doesn't take up much room) so your dogs will have it if they soil their kennels one night (ours often do, by about the 3rd or 4th day, when they're just about too exhausted to live), or if they get back in the kennel wet, or with ice or snow on them;
9) de-icer/scraper
10) Rymadil for the dogs
11) Extra boots and socks, and not back at the motel or lodge;
12) If you've got the room, extra hunting vests so you don't have to change shells out at federal or state areas -- you just switch vests;
13) Lots of map stuff -- WIA maps, county maps, and in a perfect world, satellite or other views of the areas you think you're going to hunt, so you can figure out what's got crop on the side you can't access by road, and what doesn't.
I'm getting down to some of the obvious essentials, but these are the things we've decided we're glad we have after 13 years of hunting public land. Good luck.