Traveling with a full water tank

Any manufacturer that states not to travel with FW tank (or any tank) full is aware of their faulty design and use of inferior materials. Our rig can carry over 100 gallons in FW tank and has done so for 6 years and over 40K miles. Poor water quality and lack of nearby freshwater (we boondock and drycamp mostly) are the major reasons we carry our own water. For a properly set-up towing combination, a few hundred pounds of water weight should be unnoticeable unless TV is under-powered to begin with. MPG for our towing combo does not vary more than a couple of tenths of a mile whether tanks are full or empty, speed of travel is what affects mpg most.