If you have beet juice or rim guard, I believe it has a reddish tint.
IIRC, Calcium is yellowish.
I hope you don't have this because it's really corrosive.
If it's clear, it's probably a water mix. Alcohol or Windshield Wiper fluid is added to keep it from freezing. Depending on your climate, you may want to drain out a little and test it for freeze point. If you don't have one already, I'd start the testing using a regular antifreeze tester that you can pick up at any auto store for a few bucks.
As for weight, but not knowing what you have as the liquid ballast...
Here's a chart for weight using Rimguard (beet juice, which is heavier than water)
http://www.rimguard.biz/wp-content/uploads/2014/04/Hydro-Flation-tables-2014-PDF.pdf
Best as I can tell, you have rear tire size of 17.5x24/8
On the chart, that says 55 gallons, 589# per tire using 75% fill level.
Normally, tires are filled to approx. the top of the rim.
To figure out your fill level, just look at your tires when there's high humidity or a quick change in temperature. On the outside of the tire, you'll get a dew line at the level it's filled. If you don't want to wait for such weather, use about a 1-pound (normal carpenter's) hammer and starting at the top/center, rap on it. It will sound hollow. Then work your way down one side until that hollow sound is gone. That will give you a good estimate.
If you don't already have one, consider getting a tire pressure gauge that's for low pressures and liquid. A regular one can't give accurate readings at low pressure and the liquid can ruin normal gauges.