Hello: There are many recipes for waterproofing a plywood subfloor. The best one I have found is to fix all rainwater leaks. My 66 Trade Wind had rotted floor spots in the rear, by the front door, and under the freshwater tank. When I tested it for leaks, I found 7 that I had to repair.
I think the subfloor in your Sovereign
was originally 5/8". Maybe the body is off the frame now. If the body is on, squeezing an extra 1/8" of subfloor under the c-channel would be nearly impossible.
I find polyurethane adequate as a waterproofer. Others use epoxy materials.
I don't think you need to add parameter support between the outriggers. You do need to insure all c-channel frame attachment points are solid. The aluminum body of our Airstream's is not that heavy.
The main thing is be diligent with your moisture meter and repair any leaks found right away. Keep that subfloor dry.