Seems like a logical question, why not use some fine, unused space.......Convenience and safety would be my concerns. Probably would never have an occasion that would hurt you. Just remember, though, you already have the gas tank inside the car with you. Properly vented with sound hosing, it shouldn't leak fumes or liquid......but.
Batteries were also inside cars in the old days--VW bugs had them under the rear seat, Land Rovers under the left front seat, still do in Defenders. Older vent-style batteries could and would give off sulfuric acid fumes into the cabin. Land Rover uses some vent caps and routed vents in an enclosed under-seat box with lid. And trying to either jump-start yourself, or help out someone else with a dead battery is a royal pain without some type of remote hook-up, depending on how you wire things. Sealed AGM batteries supposedly don't outgas........but there's a vent relief on them for a reason--the manufacturer knows that pressure and gas build-up has to go somewhere if excessive. Ever forgotten and left an Optima on a standard charger too long?
All this is fine and good with the vehicle in the correct upright position. However, should you get into a wreck, and you and your passengers be pinned-in, and the truck be overturned, gasoline dripping out of places and battery cables laying in the wrong places and........
Sure it's rare, what's the chances, are you a gamblin' man? What was it Clint said, "Well, Punk, you have to be thinkin' to yourself about now, do I feel lucky?" At least before you add in your battery, think about it. Lots of folks have stuffed two batteries into the engine compartment by being creative and moving other things around. That's my first choice. Anyway, my two cents.
