An RV rubber roof is quite similar to a house or commercial rubber roof; the materials are in some cases identical. A spark arrestor would be a very good idea.
If you go forward with the stove, please be extra careful in making sure it's a good setup on the porch with a proper fireproof pad and heat shielding and so forth. A travel trailer burns up much more quickly than a house in the event of a fire, and should the porch be on fire it's blocking the main exit from the travel trailer already so you'll likely be forced to use an emergency exit window. It could rapidly become disastrous or tragic. This would not be a place to cut any corners at all.
I might suggest looking into a pellet stove; they typically only need a small wall or ceiling mount vent, similar to a gas water heater, and are (at least around here) pretty economical to operate, and certainly cleaner and easier to tend than a traditional wood stove. I lived in a house with a wood pellet furnace for a year and was very impressed; it only required periodic filling of the hopper, a quick minute or two of scraping out soot from the burner pan each week, and emptying the ash bin twice during the heating season. The heat from a pellet stove or furnace is much more consistent than from a normal wood stove.
