Happy to hear you have strengthened the frame on your old Sovereign
. That is very important and will add significant value to your trailer. Take some pictures for show and tell whenever you decide to sell it.
So you will have to engineer your own waste water tanks and mounting scheme. I recommend Inca Plastics in California as a potential source for the tanks you want. They have a huge on line catalogue of all the RV tanks they have molds for, and will mold the tanks you want for you. They also have tanks with either 3" or 1 1/2" diameter drain ports that will facilitate the 3" ABS discharge piping.
If I recall correctly, the 74 Sovereign
piped wastewater to the rear bumper area and drained out the rear side of the frame. Maybe there is a 3" hole on the street side of your frame rail just before the bumper. But you don't have to do it that way. Here is a photo of my 86 Limited side drain location, and the 66 Trade Wind where I did the same thing. I think Airstream started draining waste water tanks after the axles and below the frame rails in the early 80s and have been doing so ever since. It's a handy location as most RV parks have sewer connections in this area of the campsite.
You can hang the tanks between the frame rails in the frame bays just behind the axles and plumb your drains and vents to the tanks. The drain lines will penetrate the floor and into the top of the tanks. Inca Plastics sells a nifty grommet that seals in a "hole saw" cut hole. The toilet is the least flexible fixture. It must sit over the deepest part of the tank. You will have to fabricate a tank cover, and route some furnace air to the tank compartment to keep them from freezing up. The 3" ABS drain manifold with Valterra valves is pretty straight forward.
You are essentially building a new trailer from old parts. You have a lot of design flexibility.