I use the black tank/toilet for what it's intended for.  The bathroom in my trailer is used like the bathroom in my house.
As for carrying fresh water, I've always had at least 1/3 tank, which, in my old trailer, amounted to at least 13-15 gallons.  And if I were boondocking at football games where there was no water source, I carried a full tank of 40 gallons.
My new trailer, Jayco 23RB, has 84 gallons of fresh water capacity.  It will be rare when I have a full tank of water.  However, I will always still have between 1/3 and 2/3 tank of fresh water.
I haven't seen any warnings from Jayco about traveling with water in the tank.  And I hope I don't.  If I had, I would not have bought the Jayco.
A warning from the manufacturer to not carry fresh water when traveling tells me that manufacturer knowingly makes and sells an inferior product.