Water, propane and fuel I try to keep full. Black and gray tank I try to keep empty.
There have been a few times on the road where I needed to take a nice cold shower. One time in Reno Nv where I busted a spring on my TT and had to change it in 100 degree weather.
A few other times I had to deal with flat tires and after doing that a nice cold shower comes in handy!
At 16K it makes next to 0 difference in mileage.
As the saying goes, do what you want, it's your trailer. Hell, my FIL even has a rule; no using his bathroom. :h But it's his trailer and his rules. I'm just glad I don't have to travel with him. :)