On our first two TTs, we had no trouble carrying water in the FW tank, full or partial. Our present unit does not travel well with any water on board, lots of uncomfortable shimmy with the water sloshing around. I figure it has something to do with the location of the tank, the suspension system, the wider spread of the two axles, the ultra-light construction, yada, yada, yada. I have a spare water container to provide water for the toilet while on the road, if needed.
I'm also one of those who opens the FW drain upon leaving the CG, allowing the water to slowly drain on the highway.