Never found a breather vent on my last MkIII, but it could of been in a similar place to what has been mentioned above.
On Blue Moon there is no integrated water tank, it has either been
grinded/ground out or not installed in the first place, & I find its a brilliant feature of BM, gives full 6'4" standing headroom at the foot of the steps & 6'2" in front of the sink
, I simply use a 20lt water container strapped in on the floor of the fore-cabin, & have other 5 & 10lt containers stored in lockers
cheers