Sam, regarding the drain valve....if you have the same kind I have on mine MA, it's a cheap little plastic handle that easy strips and won't actually turn the ball valve. Mine is barely visible tucked way back in behind the water bay and my installed sanicon, but if I contort around enough, I can back in to it. I took the old handle off and just use a small vise grips to turn the valve. As soon as I can get around to it, I'm going to pull the sanicon out so I can have a little better access and change the valve to something more substantial...and perhaps even reroute the drain piping so the valve is actually in a convenient location.
I can't venture a guess as to why the pump would act one way when you have less than 1/3 of a tank of water and another when more than that. My first thought was that you didn't have your selector switch to city water so the pump was just cycling water back into the tank. But it would do that no matter what the level of water in the tank if the selector was incorrectly in the position to fill the tank.
"Second star to the right and straight on 'til morning"
On the Mississippi in SE Minnesota
2013 Newmar Mountain Aire 4336
2016 Chevy Silverado 4x4