Some interesting ideas thereI've been looking at those actuators too recently @andys . For a different purpose, namely to remotely rotate the rear overhead locker by 90 degrees, so that I can make the 6 x 9 speakers face forward. Better sound that way but better loading ability when they face downward.
Have you though of....
1) Ball valve and solenoid. As found in some central heating systems. Valve fully opens without pressure behind it.
2) A submerged pump with an outlet over the top of the tank. There would no required for a solenoid at all then.
3) Taking it to the edge of the envelope here, but a small 12v air compressor as used to blow your tyres up. Pressurise the tank with air and blow the waste out when the valves open. Would need non return valve on the inlet though.

When I actually got some time at home though and started measuring, I found that space constraints would make it impossible without extensively reengineering the area around the water tank, and I just don't think it's worth spending any more time on any projects like this for now. I'm going to start spending any free time I have getting out in the van over the next 6 months rather than working on it, and then I think I'll probably end up selling it in the autumn due to external factors.
If by some chance I've still got it next winter then all my list of potential mad projects are back on again
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