Sorry to be so late getting back to the forum. Busy days.
We got the Ameri-Kart ABS Side Drain 20 gallon black tank, SKU 53146, $207.52 for our 1965
Tradewind 24'. It took a little figuring out but it works really well. We screwed it to the underside of the floor through the flange (only where it wouldn't come up in the middle of the floor space above) then we got some of that metal strapping (the kind plumbers use that has lots of holes in it) and strapped it in front to back. Under the tank we put a couple of boards that go from frame to frame across the trailer. Over that (or would that be under that?
) we put a sheet of tin that we happened to have lying around and then riveted the tin to the existing belly pan (I put butyl tape between the tin and the belly pan to keep moisture/dirt out) and screwed it to the cross boards. The tin fits tightly into the C channel frame. At the back end we plan to cut a piece of tin that will to from the bottom of the back to the floor of the trailer just to close in the space and keep varmints out. Sounds confusing but I think it will work. We haven't done that part yet.
I don't have picture of the underbelly with the tin attached and the trailer is with the repair guy getting propane lines installed. When we get it back I'll post pictures of the underbelly and, hopefully, the part at the back that closes in the space.
Hope this helps.