Aerowoood brings up a good point about the size of the tank. My grey tank is going to be my old water tank. It is only 6" tall so it will hang down from the frame, but as it is going between the axels it will not hang below them.
you can get them custom made, little more money but worth it for the black tank, as you don't want the outlet to have seams that can catch on stuff (if you know what i mean)
Talk to your local RV dealer about tanks and tank sizes. Talk to more than one because they carry different lines of tanks.
May you have at least one sunny day, and a soft chair to sit in..
2008 5.7 L V8 Sequoia
AIR # 31243
WBCCI # 6987
FOUR CORNERS UNIT