I am remodeling a '77 Tradewind. Have removed the rear bathroom which, due to condition, required destroying and removing the black water tank. Had considered adding a second grey water tank but hadn't yet found a source. Thanks for that.
I too am considering a 1.5" exit. Bought a new exit valve but haven't figured out all the transition/conversion joinery to get out the large hole at the bumper location. Actually have a plumber coming tomorrow to help figure the whole thing out. Without the bath and the black water tank I have some room to work with. It may take a while if I order a second tank, but will post updates and photos.
Mike & Laurel
Ft. Myers, FL
1977 Airstream Tradewind
2016 RAM 2500