Two great replies by @Persistent and @lcv80 but one other piece of advice is many people say it is a bad idea to leave valves open because you want all the water and junk (food bits, grease, etc., to fill up the tank so when the valve is open you have the big volume help flush the tank. When open all the solid particles can clump together and dry out creating blockages.
Once you figure out whether you really do have a blockage or a bad gauge I would suggest keeping the valves closed all the time unless you are actually dumping the tanks.
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2018 Voltage 3305
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