Great information, thanks.By looking at my pro tank, it alerted me to general rust conditions front to back under my rv. I assessed nothing merits getting rid of it but alot of work is ahead. I think that mud flat in back prevents air flow from the rear that could help air out the under surface. Now I have to find a service that will do a complete rust removal of all under. i would do it except my acid reflux makes that position not good. Doug