Under-built water tanks:
Although it sounds simple, the install is all except that. As a caravan moves forward and stops often, the tanks need to be fixed properly so they cannot come loose by these sudden and often abrupt movements. It is also important that they hang not lower then the lowest point of the underside.
The sizes are depended of the available space and weight restrictions, and an onboard pump is a must, but can be added if needed.
There are different options and most often customers choose for two tanks, one for fresh, probably mounted in front of the axle and the other for the waste water (note: this is not the toilet water), mounted behind the axle. A better, but more costly system, is the use of four smaller tanks, 2x in front and 2x at the back, but cross connected, so you always have the same weight of water at the front and the rear of the axle(s)
And, I can install a macerator pump, with one or two outlets (either side one) so you can pump the waste tank(s) empty quickly and not be hindered by higher dump stations.