Eddie. I know that a fire place would be fun, but I am sure that codes would not allow you to do this. And, lets face it, it would be very, very, dificult to keep "clearances" in such a confined space. Not only are mobile homes not constructed to the standards you would need to meet, but also the dimensions are so restricted, a fire could easily be created due to every day living.
If it is wood you want as a sourse for heat, you might investigate a "remote" furnace located in a building some distance from your trailor. I am assuming that your trailor is located in an area that is not easy to run gas, and propane or electricity is an economic issue. Ask around, but keep safety a priority.
Do not put in a fire place. I am concerned for your safety.