black water holding tank leak at 25 gals???

They don't use a wax ring.

A defective "wax ring" (actually a foam-rubber seal) would present a leak INSIDE the RV, not outside underneath.
Also, as the OP mentioned, the leak starts well before the capacity of the tank is reached. The wax ring is at the top of the tank.

I am currently in the process of replacing the black tank in my Jayco. A crack in the bottom was repaired successfully but on our last trip it leaked from the top when nearly full.
As far as I am concerned no tank should leak when full. Especially the black tank. The vent itself has nothing to do with the tank leaking but the glued on fitting for it or the toilet may be cracked. Unless they used a rubber grommet, the fittings are simply glued to the top of the tank with ABS cement. If the toilet dumps directly into the tank then I could see a bad floor seal possibly showing up as a leak. It could also be the seam around the top of the tank.
If the leak can't be located by inspecting the top of the tank in-place then it would need to come out. Not a fun job but not that difficult if there is enough access but I was surprised how much "stuff" was left even after draining. That old tank was quite heavy.

Here is a photo of the new tank and old tank. The top of the old tank is sagging and there is a water mark where the "waste" was sitting. It looked as if the toilet fitting on the tank leaked. Once I finish the installation of the new tank I plan to post it to the DIY section.

Instead of going to CW, you might consider Quality RV on Aurora. They're about halfway between Wickham and John Rodes.

It's the rubber seal between the toilet and the tank.Mine just did that and the icky stuff took out the circuit board on my furnace and my 12 volt converter.

It could be that your wax ring between the toilet and tank is leaking.

If you think the vent pipe could be part of your problem, take your garden hose up and run water down the pipe. That should clean out the vent pipe.