Whenever we go to the races in Joliet I travel with water tanks full only a 20 minute drive and buy water only if we run out. Headed to the race in Sparta KY about 6 hr drive no water hookup at CG bu...
If you have to worry about the water tank falling off because it is full. (And, yes they have occasionally.) You have bigger TT design issue to worry about.
I understand if you camp in spots that subject you TT to extreme environments you might want to be cautious about the TT water loading. But that would be just one of many rough service concerns.