Water tanks
You can see how mine were mounted under the trailer in my build thread, skersfan shuttle pod. I welded in angle iron and supported/sandwiched between the frame and the bottom of the floor. I used a water pump to get it to the shower, sink and hot water heater. Very doubtful a shurflow pump will fail, but if it did it is mounted just inside a sliding door.
I did not do skid plates. Mine is on 35's with a two inch frame and 6 inch drop on my torsion axle. If something hits them I have a lot more problems than the water tank breaking is my guess.
On my production trailers I will have skid plates though as I do not want to work on them if some does hit something.
I ordered mine from Plastic Mart and designed them just the way I wanted them. I forgot the front air vent, (DUH) but the back one seems to allow it to work. I have two 11 gallon tanks with 6 gallons in the water heater. The next ones will be bolted in.