We use 2-gallon collapsing totes which I am 99.44% sure were intended to be used to transport coffee (brewed) in cardboard boxes.
These work incredibly well, weighing about 15 pounds each when filled they hold the tent corners down very nicely.
They also collapse flat and take up nearly any space, we just carry them in the sink when traveling.
The first place I found them was
here. One of my youtube providers found and recommended it for drinking water. But even for that, the price seemed crazy expensive.
My wife one-upped me. She found
these --- a lot better deal at twice the price for what looks like gromets.
Here's the best part, they work great as weights...but they also make re-filling the tanks easy!!! The caps fit into the gravity feed hole, a quick lift of the back end so the water pours in. 2 gallons go in as quickly as a beer bong from our more youthful days

Not an exaggeration --- 2 gallons go in in just a few seconds without losing much water to spillage...maybe a cup or two? Less when I'm not showing off.
We use the 2 1/2 gallon bags, and they work perfectly well for both weight and for water tank fills. $30 for a set. We've used ours for 2 seasons now, and I'm going to replace them just because they are yellowing and starting to look a little gross -- no leaks yet. $15 a year for a solution this good is a deal!