OK you have a rearbath model, but where is the commode?
Is it along the streetside wall, or is it backed up to the rear
Is the commode resting on a raised platform? or is it at the same floor level as rest of floor in trailer?
On the 1974 models (according to the Service Manual) the rear baths with the commode on the streetside wall have a vent connection that is accessed thru the hotwater heater compartment. Of course everything may be different on a 1969.
I don't know if the vent clamp is accessable inside the HW-heater box or whether the HW-heater box must be pulled (not alot of fun either).
If the Overlander has the commode on the rear wall, I can't find any info (in 74 manual) to ID the access to the vent. But there "oughta" be a place to get a peek.
Perhaps thru the trunk compartment outside, or in the cabinet that sits to the curbside of the toilet (if there is one there).
Hold it! I just found a R&R for the '74 LandYacht (not International),
Holy Smoke it musta been written by Steven King!
Seems you have to take it out thru the inside, only belly skin cut is a small one to remove a flange directly under the termination valve.
However you do have to un-rivet and dismantle the lavatory and its stand/hamper. I believe that the landyacht is the one with the platformed pot.Again remember I am talkin '74 here.
Hell I'd be almost wishin' for the Cesarian !
Now all this may be in vain, and the leak may be something else entirely but who knows maybe you can see part of the black tank top there?
As to bellyskin opening, it prob'ly depends exactly where the tank is located. Cutting and then patching with a new (larger)piece may be the most practical route. Or drilling out the rivets and rolling back a flap is preferred if circumstances permit.
I am amazed that no owner of a 1969 has come forward as yet, but I hold out much hope for you guys.