Gary, my main reason for changing tanks is the original tanks will eat up all my basement storage.
I don't want to hang out past the rear axle more than 6', so it's going to be a little tight under there.
And since I'll be changing a lot of the original factors the engineers used when they designed these trucks, I explore what effects changes will have. I'm shooting for a 25,000 lb conversion compared to the 80k the truck was designed for. There is a good chance I'll also be removing up to 600 lbs from the rear by changing out the duals for super singles. To be honest, I'm considering changing my mind on using aluminium for the RV frame because I'm worried I might not have enough weight to soften the suspension. IE. the whole ping tank problem that started this thread.
And FWIW, I think a poly tank would be a better solution for fresh/gray water anyway. It can be formed to use space that otherwise might not be useful. I'm hoping the numbers work out to fit a pair of 120s behind the rear axle, partially pushed up between the frame rails. There's no way you're going to put potable water in a used fuel tank, I'd even be hesitant to use a new one if I wasn't sure I could get it clean. And if you use it for gray it's going to be a pain to dump unless you move your toilet and black tank near it.
-lee