prevent fresh water from freezing

The fresh water tank is inside the camper shell, so logically, if you don't freeze, the water won't either, lol.

Seriously, warm the camper before you leave and close all windows to seal it and you will be good.

We camped at below zero in Yosemite and everything went well. But then, it was only -10C.

think this threw real good, is a broken buried water line worth it.use bottled water to wash with.

Vodka is for drinking.
The rock salt idea is probably a good one.
The pink stuff is pretty harmless if diluted and you don't drink it.
I think you are on the right track.
Figure out a way to insulate the bottom of your rig and enclose your plumbing if it did not come that way.

There are a lot of nuances to really cold camping. I wouldn’t use salt in any case. I think you need to do some searches for do/don’t and not just focus on what to put in the tank.

You could consider putting a 12V heating pad on the side of the tank, however, the tank being a large mass of water will likely not be the first thing to freeze. A small water line in the least insulated area will freeze first.
It's more about knowing how cold it is getting and what your camper will endure before freeze up. You're not going to lower the freezing point of fresh water and keep it as fresh water.

80 proof vodka freezes at -11F, so for 0F we can estimate you need at least 60 proof in your tank.
When I like the idea, I think running heater will become way cheaper.

Not only will RV antifreeze not help if diluted, you will NOT like trying to shower with RV antifreeze in your water. Since my camper does not have a winterizing kit I have to dump a gallon of pink in my fresh tank each fall and circulate it through the lines. There's maybe a cup of antifreeze left in the tank.

Anyway, one spring I thought I'd flushed the tank adequately but apparently not. Got in to take a nice hot shower and that tiny bit of RV antifreeze made my eyes sting something awful. Had to endure a 3-day trip getting my eyes burned out every morning. It was either that, or stink.

If your fresh water tank is inside the camper shell and the camper is reasonably well insulated you shouldn't have an issue as long as your water lines are also inside the camper shell. We have camped in zero weather and never had an issue. If you heat at night while occupied you should be good during the day.