The more important concern of towing with a tank filled is the effect it may have on the loaded TT tongue weight. If the TV/TT combination has been set-up (weights, WDH adjustment, etc.) based on empty tanks, then depending on the particular tank location (front/rear of TT axles) it may add/remove weight from the TT tongue weight thus possibly effecting TV handling characteristics of the TT.
Best way to confirm the effect is a visit to the local CAT scale: http://www.jaycoowners.com/showthrea...igh-Your-tt-tv
Traveling with a few gallons of water in the fresh water tank should be fine for those occasional stops along the way.