Tom,
I had to magnify to 250 and (I think) I see a fire extinguisher on the second jeep hanging off the left side or the tub. There's a lot of images in the section on fire jeeps but I haven't seen (or can't recall) one in that location. There's a good image of a bottle sitting on an extended running board on the back of a truck, but I would suspect on our jeeps hanging items off the tub, like the spare tire, tend to get caught on trees, etc. from lack of clearance. There was one other image I remember seeing that had a large bottle up front just behind the passenger fender but the image was from the driver's side so not much showed.
Mounting it internally in the back would be best from the perspective of being stable and secure, but you give up valuable floor space. Not knowing the actual size of that bottle, I suspect a beefier bracket than the typical "wall mount" type would be better and more secure since the jeep won't be sitting still.
Will this be functional (heavier) or just for show?
Added: Thanks Chuck! That did help.