Don is right...
It's the animal fats in waste that clog up internal holding tank senders. Vinegar won't work, but a strong solution of detergent and water will help some, and MAY remove all the gunk if it's strong enough and allowed to soak long enough. But I wouldn't count on anything except pulling 'em out and cleaning 'em in detergent and water by hand to keep 'em working more than a few months again. And as long as you have a gauge that has internal senders, you can look forward to removing 'em and cleaning 'em every 2-3 years, if not more often.The Profile tank monitors http://ferriellosales.com/default.php use senders that are attached to the outside of the tank, so they never need cleaning. Their senders are compatible with Wema and SeaLand panels (also Snake River) and can use the same wiring, so you don't have replace the whole system or even do any rewiring...just swap out the senders. And the price is VERY reasonable.