my 5lb fire extinguisher along with a fire blanket is in my tongue box. I see others mounting them inside the bed area of the trailer or galley - my logic for the tounge box = it's located at a halfway point 'if/when' needed between the tow vehicle and trailer, along with if I was to have a fire = the plan is to "get away" from the fire - thus having the fire extinguisher in a 'neutral' 1/2way middle point for easy access 'if and when needed'. (also carry a 2.5lb between just inside the driver's door/seat).
I have never liked the thought of having to 're-enter' a burning 'bed/galley area' trying to retrieve a pre-mounted fire extinguisher if that area is on fire - would rather be able to get out & away first - then return to fight the fire if able too so...... (when during our group trailer outings/crawls - I would normally pull the 5lb 'out of' the tongue box when camped, hooking it to it's outside hook mount on the box, then informing others in our group of it's location for it's use if need be..........)

the 'fire extinguisher' decal showing it's location and red mounting hook can be seen here on the side of my tongue box.....