I had the exact same issue 2 weeks ago on my 328. In my case the tank would not drain at all. I tried using the black tank flush to clear the blockage. When I did this air pressure built up in the tank and when I opened the flush valve water and air began rushing up through the toilet.
I had to have the tank pumped by a septic company. During the pumping there was a loud pop, presumably the blockage, and the tank was quickly drained after that.
I am convinced there was a blockage in the vent that was stopping the black tank from draining and causing the pressure build up.
It has only been 2 weeks but I have not had any further issues draining the black tank.
Whenever I hook top now I always use the black tank flush to partially fill the tank with water.
(Ron & Karin)
2015 Georgetown 328TS