I usually have my Marlin guide gun under the seat when running around at home and work in rural Alberta, although it comes in the house with me most of the time for maintainance reasons.
I don't like leaving a firearm in the truck when I'm in the city, just because I'm worried about having it stolen. If I was going to keep a dedicated 'truck gun' in the vehicle I'd go with either a full stainless rifle/shotgun or something cheap and simple like a norinco, and probably store it in some sort of oil-impregnated soft case or wrap behind or under the seat.
Is it legal to leave an unrestricted firearm in the truck - Yes, as long as you follow the regulations. Heck, lots of people on here will tell you it's illegal to drive around with a shotgun or rifle visible in a back-window rack, but that's BS too as long as you're in the truck.