We have four all together.
There is one that came with the fifth wheel right at the entrance door, I keep another large one under the bathroom sink in the bedroom, easy reach from the bed. There is one behind the drivers seat in the truck and one in the basement storage.

best bet is to shop for the best deal. In my 39 class a i have one at door, one near stove and one in bedroom. Some may think its overkill but i am not running from the bedroom to the door to fight a kitchen fire. The one in the bedroom is large i would rather have to much then not enough, My fat ass doesnt fit threw the emergency window in the back