Interesting. My tank has a slightly sloping bottom and a 'low point' built into it where the drain valve is. I think the hose from the tank to the pump is attached to same area. After draining tank and running pump, nothing was coming out of faucets. Perhaps since it could not develop pressure, water was running back out of pump into strainer? Once most of the water is out of the pump, it will no longer have much suction, as it is no longer primed. I didn't run my pump as long as you did though, as I was afraid of running it dry might damage it.
2019 Rockwood Geo Pro G19FD w/off road package
2015 Ford F150 XLT Super Cab 4x4 V8