I had a BOAB 40 litre hard plastic tank just behind the front seats in the foot well. Taps on either side with hoses. They are strong and you can stand on them. Only problem is that there are no baffles so if I used 5-10 litres and was driving over rough rocky/potholed tracks you could feel the weight of the water transfer from one side to the other as the vehicle moved. Very disconcerting.
For general touring would not get that problem. Do the bladders have the same problem?

I would now use plastic jerry cans or those 10 litre boxes you get from the shops. I used those to drink from while driving to the Simpson so I could conserve the water in the BOAB until I hit the desert. I hindsight I could have just used those.