Locate the furnace vent on the outside of the RV and then go look on the inside in this same location. Also there will typically be a fresh air intake vent inside the RV on the wall, cabinet, or whatever they covered the furnace with.
If you dont know what the furnace vent looks like. Have somebody start the furnace while you are standing outside. You should hear the blower motor start to ventilate the system and the you will hear the furnace start. Just listen for this and you should be able to locate the vent.
SlimTim is correct. You should be able to see either one or two two inch holes on the outside vent. There should be a removable cover to where you should be able to pull the furnace out if it needs service.
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