I buy a TT with the anticipation of using it to full capacity. If the water tank cannot be used at full capacity while towing I would expect such a disclaimer in the owners manual or other literature.
Obviously some designs do not work, but that does not mean they should not work.
If you have questions call or contact the factory with the VIN number. That is what I did, to discovered that I could not walk on my Dutchmen Lite roof. I was suspicious because there was no ladder.
I always carry about 2/3 of a tank of fresh water, mostly because we camp with electric only but a couple of time the camp water did not taste good. If I am going where there is no water available, I will carry a full tank.
I have never noticed any significant towing difference with a full or empty fresh water tank.
