I never tow with water in the camper so I don't know about "Sloshing". All of the CGs that we have been to have water available somewhere in the CG. We just fill up and charge toilet when we arrive if we are dry camping. If the CG doesn't have water anywhere (which I've never encountered), I'd just find a place close by that has water.
I carry an empty 2G plastic jug (with a large lid) that I fill up and use for charging the toilet. I also use that jug as my private......ahhhhhhhh......dare I say it????....uranal while camping. I just put potty chemicals in it, (just like the toilet). It saves a lot of room in the camper's toilet for the rest of the family and I can take it to the restroom and dump it in the toilet.
The trick is to remember to put water in the tank
before you set-up.....
.