were the vent is outside it will be directly behind that. maybe in a or under a cabinet or step? no outside access but that vent will tell you right where it is inside.
My linen cabinet does not go to floor, there is a panel at the bottom furnace hides behind that panel in my unit
The shower is inside behind the vents. There's no access panel inside the bathroom. Hopefully, the shower doesn't need to be removed to gain access to the furnace!
I looked at your floor plan on line and saw a pic of the unit that showed the furnace vent next to the water heater. My guess would be in your bunk room on the wall touching the bathroom there is a panel that goes under the shower or in your pass through storage there is a panel. but that is just a guess?
ive never seen a unit that did not have furnace access but I haven't seen everything.