in the fire service we have the full/empty rule for all non baffled water tender trucks (1000 gallons +) but the smaller tanked trucks dont get the same effect. IMO the tank size and dimensions as well as location on the vehicle ( above/below the center of gravity, all play into the safety factor of traveling with a partial load of water on.
our tank is approx 12" tall 18" wide and 3' long ( 35 gallons IIRC), i wouldnt worry at all about the sloshing effect from that tank being half full. the additional weight on the tongue of my TV is more of a concern for me.
but the most important thing IMO is what you feel, if you arent comfortable, dont do it.
best of luck.