Greetings Seaboldarch: Welcome to these Forums and the vintage Airstream hobby. You jumped right in the deep end with your 73 Sovereign
31. I enjoy the vintage Airstream hobby and spend last winter under a friend's 76 Sovereign
. Maybe I can offer some of my experience with waste water tanks.
Airstream was big on rear baths way back when. They offered a big, luxury bath with tub and hanging closets that I'm sure sold a lot of trailers. But there is not a lot of room for two waste water tanks at the rear of the trailer. And Airstream drained the tanks out the rear of the trailer next to the bumper.
By the mid eighties the rear bath was waning and a mid bath layout was becoming more popular. Our 86 Limited 34' has a mid bath, with the bedroom at the rear of the trailer. I think it is a better floor layout as you don't have to walk through the bedroom to get to the bathroom. Sovereign trailers in the seventies had a mid bath option, see link below.
I installed a 30 gallon black tank, and a 27 gallon grey water tank under my 75 Overlander 27'. I kept the rear bath layout. I threw away the original tanks as they were shot, and too small. I drained the bath sink into the black tank to more evenly distribute the waste water. The shower drains forward into the new grey tank that I mounted behind the black tank. Two photos below show the tanks.
RV waste water tanks are readily available. But it is totally dependent on where your toilet is going, and how you will plumb the waste water drains. Modern convention is to drain the tanks out the street side of the trailer as that is the campground convention in most places.
WBCCI #8607 VAC Region 11
KnowledgeBase trailer renovation threads: 69 Globetrotter, 76 Sovereign, 75 Overlander, 66 Trade Wind Such fun !