If tank is old and sat for quite a while then as another post mentioned it may be 1/2 filled with sludge. As another post mentioned make sure your vent is clear. Then I recommend 3/4 filling to tank with plain water and getting into some choppy water for a couple hours at least. That will likely loosen the sludge by internal-tank wave action When you find a good pump out location (with correct fittings) evacuate it, fill on the spot with water and evacuate it a second time. Hope that helps!
BTW - What gallon tank do you have and what type toilet(s)? It may be filling quickly because it is not too large, and/or, because the toilets pump too much water during flush. Let us know about those two instances as well as how it turns out for you.
Happy Boating Daze! - Art
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