If you're just trying to empty the main tank (the valve on the soft hose, in front of the wheel, as opposed to the valve on the PVC-looking pipe) you shouldn't need to open faucets. The fresh water tank has an overflow hose that will not allow a vacuum to form under normal conditions. If that valve isn't draining, I'd wonder if there were some sediment/crud blocking it up. If that valve started draining when (and only when) you turn on the pump, then something is wrong. I can understand the other valve (coming from the pipe, hot water tank) not draining until the pump is on, because you may not have had any water in the hot water tank until you pumped some in ... and then out the drain!
I think I'd like some more information/details. A pic of which drain you're opening would help as well.
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2009.5 2720SL
2006 Toyota Sienna
2018 Audi Q7
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