I have added additional water tanks to our current and previous two trailers. Between the axles is the perfect location weight and balance wise. Forget worrying about " road debris". I will not purchase a trailer with underliner and my tanks have always been exposed, including black and gray. Thats making a mountain from a molehill. If you hit something large enough to damage a tank its going to also tear right through coroplast and probably take out all the ABS plumbing and some other things as well.
I use a 3 way valve to switch from one tank to another. You can only have so much line between each tank and valve and the valve and pump. The pump manufacturers say 6' but I have had 10' and the pump will prime and siphon when switched.
If you can lay your hands on an Alpha Systems tank they are the best I have found. 39 gallon with depression for pickup and drain, really solid for a blow molded poly tank. They were on Amazon for $162.00.
Alpha SystemsThe Amazon pictures do them no justice. Follow link and click on either VB28546W or VB28547W
Cad Drawings
Too many geezers, self appointed moderators, experts, and disappearing posts for me. Enjoy. How many times can the same thing be rehashed over and over?