Just thought I would retouch on this topic, as winter approaches, and a lot of people tie thier wheeling into camping, which almost always requires a big fire in the winter.
I personally carry one BC Marine Grade Extinguisher in the rig, mounted on the floor right under my drivers seat, for easy access. I also carry one in the tow rig, and one or two on the trailer.
Overkill? Probably. But if you have a fire, you'll be glad its there.