Having experienced this horror firsthand I would defiantly recommend a minimum of two extinguishers, a small one that can be accessed by the driver while seated and a larger one mounted in the back. In my case I drove (well coasted really as I'd lost power) for better than a 3/4 of a mile with smoke and flame roaring through the vents before I could get it safely off the interstate in a emergency pull off bump out. There was zero chance of saving the truck but preventing the blaze from spreading or causing injury to others was the paramount concern at that point.
In the heat of the moment a gas type could be worse than nothing at all, panicked breathing in a closed cabin combined with smoke and a sudden influx of co2 in not a recipe for success. I'm an old hand at life or death and didn't panic but if I had had a small gas unit I might have lite it off without thinking, it's just reaction at that point. Better to have it thought out in advance and prepare for worst case. I'm leaning towards a foam agent for rapid, non-targeted release and it's ability to stick, mostly to me so I can egress safely.
Out back it's personal choice though dry chemical in a 5lb size is my ideal for a fitment and use, big enough to provide a decent emergency blanket but in an easily stow able configuration. If I was doing extensive trekking off the beaten pass I might opt for larger, once again more as a preventative to incidental damage to the environment than anything. Trust me, by the time the flames are licking out the wheel wells there's no chance of getting the hood up, that baby is toast.