Aqua Magic V Problem

OK, down pipe not straight. I will try to explain this better. Tank has NEVER GOTTEN FULL BEFORE. As stated I dump every Friday even though it isn't full. Always dump 3 or 4 full toilet bowls before opening the valve. I have a clear plastic elbow on drain, sewer hose attaches to that. When I pull sewer lever the elbow fills and water gushes for 2 or 3 min, before slowing down. then drains for quite a while before getting down to a trickle. Then I turn on flusher till water comes out clean and shut it off in two places. So even if you figure I only got 25 of the 50 gallons dumped, where did all that water come from?? I figure I got done dumping at about 2pm, so from then to 6am we probably flushed maybe at most 16 times. it was only no.1 so maybe at most 16 quarts of water. That's two gallons of water. How did the other 20 or so gallons get in there to have it full to the point of causing a geyser coming up from under toilet when I flushed at 6am/? Still a mystery to me! I should add we fill bowl when dumping no.2.