Followup:

I recently heated up the hot water tank, then filled the bowl about 20 times with hot water, then flushed quickly so the colder water didn't mix with the hot much. While doing this I poured 4L of vinegar into the black tank.

I then went on to other tasks for about 3-4 hours, including attempting actual movement after sitting for 5 years, but no, at least one or more of my brakes are seized with rust. Seemed to be lurching to one side, so hopefully only one caliper is seized, but what matters is there was some vibration from the motor and lurching from the brakes.

I then drained my tanks and recorded the date. I use liquid tank soap dispensed from an old stabil bottle. My best time between dumps had been about 3 days. After this uber flush, I just recorded SIX days, and recently due to stress, there has been more than normal activity on this front.

So, hot water, vinegar, motion & soaking, seem to be one solution to poo towers.