We have a second tank we got made up in the Ute. And carry 1 20lt drum we use for showers that can be topped up at creeks etc.
In our previous 4wd, Jackaroo, we also had a 80lt tank in the back of the wagon.
Advantages of having them in the 4wd is that if you camp without the trailer you have water. If you are set up at camp and need to go and get more water it's can be easier to just duck out in the 4wd top that up and then transfer water across to the trailer. Also often the tanks go where the spares are carried on trailers so then you have to find a place to carry the spare.
We have a pump on the Ute that can pump water in from creeks etc and out across to the trailer. We also have a sediment filter we use before filling any of the tanks, trailer or 4wd. And a twin filter on the trailer.
There are plenty of places out west where water can be hard to get, when it's in short supply we don't feel comfortable asking to fill up. Also we have been to plenty of places where the water doesn't taste so hot, so having that extra water in the Ute can be very handy and only fill up all tanks when the water is good and in plenty of supply.