You can search online for tanks. YOu can use the plastic tanks for black and grey . I got A very nice fiberglass replacement for the original from inland rv. I went to my local rv guy with dimensions I wanted for my grey. He found one pretty close. He spun the fittings in for me and i strapped it with some heavy gage straps with holes in them to the frame. I looped it back on it's self and put bolts through the holes. Never have had a problem yet. And if there is any problem I can get it off without ripping out the floor I cut a hole in the belly pan and then made another cover for it so I can just drop that without taking the whole belly pan off. It hangs down about 3 inches bellow the belly pan but above the axles so nothing can hit it.
I know some people like to keep it in the frame but I wanted a 40 gal one,
They started putting grey tanks in around 72 I think. Someone will know for sure. They weren't very big though.
The only thing I didn't do was put sensors in to keep track of how full the grey tank is. That why it's the same size as our fresh. When we are out of water it's time to dump...