Regarding towing with the tanks full, it depends what the road is like and what else is in the camper, what destination you are going to depending on what their water is like and do you drink the water from the tanks.
A lot of different scenarios which will have different answers.
Personally I never drink water from my tanks, I have 2 tanks totaling 140ltrs. I only use the water for washing and showing and take water bottles instead and a lot of beer that has water in it.

To give you a scenario I will half fill both tanks on my camper for the first leg of the Cape trip and use the front tank first as we will be tackling the old coach road first which has hills and rough tracks.
I will top the tanks back up at Dulhunty River again the half full to even the weight. Once I get to Eliot falls I will top up again for 2 nights on the west of the Cape as there is no fresh water there.
Once I get to Seisia I will only use the rear tank as there are huts with water. On the way back home I will only have the rear tank with water.
If I had to drink the water from my tanks I would boil it first.
Every one is different. Yes you may use a little more fuel but how much is too hard to measure.
What works for you is the best way.
Mark