Hello from Colorado: You are embarking on a big project for sure. There are lots of vintage enthusiasts here that have done the same type of renovation. Air Forums can help you.
Maybe you have done your assessment and have developed a project plan. The fact you intend to pull the body off the frame means you have subfloor and frame repairs and restorations to do.
And you have to decide on your interior layout. Your new gray tank needs a vent so it will fill and drain. You may want to improve the old black tank too. Toilet position is a key factor in the bath design. Bath design is key to the kind of tanks you want.
You didn't mention if your Sovereign
is a rear bath or not. I assume it is as most were. But late model Airstreams have the bedroom in the back and the bath in the center. This design makes better sense to me than a rear bath. Some folks take on a complete remodel this way.
I would recommend you wait until you get the body off and frame work done before you order tanks. Plastic tank lead times are not a project killer.
I have a 75 Overlander twin bed rear bath trailer that I'm doing some serious renovations. My trailer had rear end separation (likely yours does too) subfloor rot in the rear, bad axles, old appliances, too much weight in the rear, and the list goes on. I have installed a larger black tank under the existing toilet position, and a much larger gray tank. My trailer is a "rear drain" and I have kept that layout. My wife's trailer is a center bath with a below the frame rail drain manifold like most modern Airstreams.
So the plan dictates the tanks. How will you mount them, how will you measure the water level in the new tanks, how will you plumb the drain and vent lines, how will you drain the tanks, how will you heat the tank compartments, and how will you fabricate the tank "pans" so the tanks are protected from road debris. I'm sure I forgot some consideration needed.
So possibly there are a lot of unanswered questions to your project before you make a tank decision.