You want it in a secure place, close to the exit, and you want as big as you can get.
Just inside the door is generally the best place. It's available for use both inside and out that way. It's at the exit so you would naturally go to the safe exit, get the extinguisher and then decide whether to go back and fight or to flee. Safety of the user is important.
The bracket that you mount it with should keep it from getting broken. Hopefully it was just an odd bump that dislodged it. If it's really in the way, perhaps move it back or sideways some to a better location.
If you have a fire, bigger is better. I carry 3A 40BC extinguishers. I buy mine at Costco, the lowest price I've found. These are commonly called the 5 pound extinguisher, instead of the more common 2 1/2 pound size. There are several reasons for this. One, twice as much agent. Two, a hose to direct the agent around a corner, thru a slot, under a counter etc. Three, it's big enough to break a window if needed to gain access to a fire. I have 3 extinguishers in my rig. One in the bedroom, one by the door, one in the propane compartment. I have a MH so your trailer may need some alteration of this placement. The propane compartment which by law is not ever locked gives me rapid access even without keys to get inside.
Of course you probably remember to shake the units monthly to keep the agent from packing inside, check the gauge - tap it with a finger to make sure it didn't stick, familiarize with the locking pin, aim at the base of fire, sweep agent across the base, never "practice" by shooting a little of the agent out, and all those other things.
Here's hoping you never use it.
Ken