Appliances that remove air from a structure will require some sort of make up air. If the structure contains enough cubic feet of air and consequentially adequate infiltration, then you may be good to go. If not, then you should provide a dedicated source of make up air. The problem with small structures is not only the lack of existing air volume inside, but also the lack of sufficient infiltration sources. The issue with combustion appliances is the requirement for combustion air as well as potentially, make up air. Many mobile homes built over the last couple of decades have a make up air system included with the forced air heating system. This may be verified by checking the furnace blower compartment for an 'attic vent' in the top of the compartment. I can not speak to the adequacy of this vent.