Most of the water pumps do have an internal check valve. As Porkchop said--at 85% your tank is obviously full/near full, so gauge is off. Possibly you have a tank with the vent installed too low for 100%, or gauge is off....the use of a foam rubber plug (ear protector, or, I am using a foam dart out of a play gun) will allow air in/out, but stop most water movement.
You seem to be resigned to the situation, so just keep on keeping on....
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2016 Tiffin 40QBH
2015 38RSSA--traded
2005 36TK3--retired, but it was a good one
2017 F450 KR
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