Since we discovered the holes in the tank, it wasn't properly pumped out last week, when we thought it was.
That doesn't make sense. Holes in the top of the tank should have provided the source of air needed for a complete pumpout. All tank vents--water, fuel and waste--have two functions: 1. to provide an escape for air in the tank displaced by incoming contents...otherwise the tank--the whole system--becomes pressurized...and 2. to provide a source of air to replace contents as they're pulled out...otherwise the pump will pull a vacuum that won't allow more than a gallon or two to be pulled out...which is the reason why it's essential that tank vents never become blocked. So even if the vent IS blocked, the holes would become the needed source of air.
I strongly suspect there's another explanation: If the pumpout fitting is on the top of the tank, there has to be a pickup tube on it inside the tank that goes to the bottom. If your tank is metal, any pickup tube is certain to be metal too...and if it is, I'd bet real money that urine has eaten through that tube in multiple places and a portion of it may even have broken off. When the pumpout has pulled out enough to drop the level in the tank to the highest hole in the tube or to the end of what's left of the tube it starts pulling in air, preventing it pulling any more out.
--Peggie
"If you can't explain it to a six year old, you don't completely understand it yourself." --Albert Einstein