When we first bought our camper, I found that I lost fresh water suction way before I ran out of water. When I checked under the camper, I found that the tank bottom sagged enough to allow much of the water to be below the suction line.
I also found that the tank was only held up by resting on an angle iron on the front and back of the tank. As the tank sagged, the edges moved inward, effectively pulling it off it's supports! It only had about 1/2" of tank sitting on the support on each side. I now understand why the dealer told me not to travel with a tank of water. That's their solution to poor construction I guess. Anyway, I added several steel support straps so I COULD carry a full tank of water safely.