If you're not hooked up to sewage, then I'd guess you not hooked up to water either. So then I'd guess you are using the FWT and water pump. So then I'd guess that you don't hear the water pump running abnormally, which then I would think there is no water running in the turlet.
Are you sure you are taking 30+ gallons out each time you're hauling sewage ??
What it sounds like to me is that you have been cursed w/ the "BLACK PLAGUE"!!
Somewhere in the tank there is a pile of solids that is decreasing your tank capacity by a whole bunch.
On the other note, the foot valve not sealing has nothing to do w/ your problem.