Worthwhile to insulate/enclose underbelly without heating?

We do not winter camp, so take this advice with a grain of salt. Our old trailer did not have an enclosed underbelly nor tank heaters. When I was going to have tank heaters installed to winter camp (which we never did), I was told that I only needed a tank heater for the fresh water and to use salt in the black and grey tanks to prevent freezing.

I do not know if this was work though. Seems like the salt would cause some damage but maybe I am wrong. Other people seem to do it I guess.