Most towables are not capable of carrying much water in the first place. JMO, but towing the extra weight is not the problem. It is the stopping it that one needs to be concerned about & to a lesser degree getting it moving.
We have a 100gal capacity & don't give the added weight much thought unless doing some serious mountain pass driving.
Sometimes weight is a good thing. Driving in windy conditions the more weight & the lower it is, the better, the safer you are from getting a blowover.
Sometimes you need to fill & drive with it. For example, we do dry camping bluegrass festivals so we fill right up at last CG before festival. Headed for Albuquerque Hot Air Balloon Festival this Oct for 5 days dry camping with 3 people onboard. Will fill up before that one.