All you need is a tape measure. Measure the distance from the coupler (ball location) to the midpoint between the trailer's axles (or to the axle on a single-axle model). Measure the distance from the place where you will be adding weight to the trailer axle midpoint. Divide the added weight distance by the ball distance - that's the fraction of the added weight which will be removed from the tongue.
- It looks like about 15 feet from axle to coupler on an Escape 21', and 7 feet from axle to the face of the bumper. If you hang something a few inches behind that on a rack, for every pound added there the tongue weight would be reduced by half a pound, because 7.5 feet is half of 15 feet.
- The middle of the dinette of the 21' appears to be about 5' behind the axle line, so for every pound added there the tongue weight would be reduced by one third of a pound, because 5 feet is one third of 15 feet.
It really matters specifically where you're adding weight. "Behind the axle" could have very different effects, depending on how far behind the axle.
The freshwater tank is probably centred less than four feet behind the axle (so, tongue weight reduced by about 1/4 of water weight) but it would be easy to measure.
There are owners who have measured the effect on tongue weight of adding weight to the rear, by actual scale measurements, in more than one model. That's actually less accurate, but it's reassuring to see that the theory actually works.
Maybe they'll re-post their findings.