You can get a fairly inexpensive pistol safe and bolt it to the vehicle and it should be pretty secure., heck an ammo box and a great lock out smarts most dumb criminals.
My truck gun when I'm going to town is a 9mm with 3 extra 15 rd. mags and a pocket 22lr, once general gun hunting season opens my truck guns are a Mossberg 12 ga. pump and a Rem 74 in 30/06 or if headed to the river the Remington is replaced with a Marlin 30/30