Hello, I've done similar things on a few 60/70'sAirstream's but I'll focus on our '68 Overlander. One of my main goals is to get as large a gray tank as I can so out comes the black tank (ours was fine) and then I source the largest tank that will fit the opening. I believe inn the '68 it is 35 gallons. This is kind of funny because the only thing that goes in it is the kitchen sink. We have basically eliminated the bathroom and in effect created a rear bedroom. The toilet we went with is a porta-potti "Curve", sleek and somewhat stylish, electric flush, fairly compact and tall enough to be comfortable. It stows in a closet for travel and when not in use. We also don't have a shower in the trailer, there is an outdoor one. So, kitchen sink, porta-potti, outdoor shower, works for us. If we need to poop in the trailer (under 5 times is 3.5 years) we use the bag method popular with van folks and treat it like dog waste. Bottom line is at some point someone has to deal with waste in some way, and we all know who that usually is. We have done multiple moths ar RV parks with this set-up. And yes, replace all that copper with pex.
Good luck, Mark D