Originally Posted by Bob K4TAX
I agree, that area behind the screen looks to be the furnace vent and air intake. As to the rest of the rusted metal, looks to be like someone's latter add-on, suitable for the junk pile.
I'd take it off, clean the area of any rust using Naval Jelly, patch the holes with marine epoxy and forget about it.
After doing some checking, that is the way Forest River configures the furnace vents on the Sabre model. Perhaps it is configured that way to allow removal of the furnace from the outside?
Anyway, to the OP: That is in fact your furnace vent. Your water heater is to the left of it. You'll probably have to contact Forest River for information on a replacement.