Smelley exhaust from hot water tank

Bees or other vermin like to nest inside the vents for the heater. They die when you light it up, but their little bodies decompose and cause smells. Maybe that?

Also, if your heater is near by your batteries, the movement of air could be pulling out a boiling battery smell. If that's what it smells like...rotting eggs. If that's the case, could be your converter is bad and over volting the batteries.