Animals Guide Placeables Liquidmanure Tank Extension (FS22)
The "Liquidmanure extension tank small" is just that, you cannot put in or take out slurry from it, it simply extends cows / pigs barn slurry capacity by 500k liter amount. "Liquidmanure extension tank large" is the same, just a very large and with 2.6m liter capacity. That is very impressive capacity coming from giants, I cannot imagine what they were thinking about such placeables for elmcreek alike little girlie men terrain lover theme 
When "Liquidmanure extension tank small" was connected to my dairy cow barn and there was 51k liters of slurry according to animal menu and FS22 Production Inspector mod, I could not "Start Refilling" from the normal slurry output pipe by the side of the cow barn, nothing happened by "Liquidmanure extension tank small" either which doesn't even have any triggers on it. Once I sold "Liquidmanure extension tank small" then all my slurry disappeared along with the sold placeable, I got money refund (harvest income) from the slurry. NOW when I time accelerated and drove tractor under cow barn slurry output pipe, it worked OK, slurry was refilled into tractor tanker trailer. Humph so "Liquidmanure extension tank small" is not working, it somehow disables cow barn slurry output pipe... maybe this is literally so easy that you have to buy "Liquidmanure tank" placeable first, OR, simply not buy the infamous "Liquidmanure extension tank small" in the first place. Hmm very strange indeed, guess this extension somehow hijacked cow barn slurry output that it bugged out as there was no MASTER slurry tank present, who knows. Anyways it works with simple basic liquidmanure tank so use that only. You can use extension tanks once you have "liquidmanure tank" MASTER placed first, in my case I had liquidmanure tank with already slurry on it, then placed liquidmanure tank large and my MASTER tanks capacity increased by 2.6 million, so it works nicely.