I would think they put the tanks on so a person can use them. Not everybody camps where there is a water hookup. I travel with a full tank of water.
As far as porpoising, that could be caused by the weight distributing hitch not being set correctly, or, not having enough tow vehicle to handle the trailer.
You can make your own scale pretty easily. here is a link.
If you post the make, model, and year of your trailer in your sig. along with your tow vehicle, people will be able to help,and answer your questions better without having to ask for that info.