What is a truck gun? Maybe there aren't any wrong answers. And maybe depending on where you live is what makes you decide what a truck gun is.
Many of the handguns mentioned could be hanging from my belt anyway, so I don't see them separately as truck guns.
Out west, where we could be out on the boonies, away from civilization regularly, a carbine of some sort is most often my choice as a truck gun. Or even an 18" barreled shotgun with sights.
Targets of opportunity abound while we're up in the mountains. So if I'm up in the sticks during hunting season, something capable of shooting a deer beyond handgun distances are in order. Simultaneously, grouse are abundant, so that's where a shotgun with sights could pull double duty. The rest of the year, a varmint-capable gun could be the requirement.
And being up in the mountains, it's not unheard of to get stuck up there and have to spend the night. A light and quick handling carbine suits many roles.
Short lever guns don't take up much room. They're easy to grab and easy to pack. And although I don't purposely abuse my equipment, riding around in the back seat of a pickup also takes a toll, so stainless is important to me. It's often soaking wet out here. Get in the truck wet, turn the heater on, and you've got a very humid environment. Stainless is more resistant to this treatment.
I very much like a .30-30, .44 Mag, or even a .357 carbine.
In the off season, a pistol-cartridge carbine is handy. And in the seriousness of deer or bear season, a Gunsite Scout Rifle in .308 covers the bases well, although my leanings are still towards a lever gun.