My guess is that it's behind and/or below the microwave unit area. One give-away is to locate the Webasto's furnace exhaust outlet on the hull's outside, and this will give you some idea whereabouts the furnace is located inside the boat.
The Webasto is under the passenger seat just outboard of the microwave. You can access the unit by removing the seat. If you look underneath the seat base you'll see the mounting hardware.
