I would attack this with an oscillating multi tool. Metal cutting blade flush against the sink base and go at 'er. Like like you have room under there.

All 3

3/4" ply of any kind should be just fine. The more dense the better. You just don't need pressure treated, which is treated against moisture and the outside environs.

Rip some strips from the edges of your plywood for the cleats. Make them, I dunno, 2" tall and screw the strips to the studs. Shouldn't use pressure treated indoors.

Damp is fine. For future use, get a brush & roller spinner. They're like $10 at the local big box stores. They spin your brush real fast and it ends up damn near dry.

The fiberboard siding on the outside of trailers has absolutely ZERO strength for attaching anything to. It's heavy paper covered in paint to keep it dry.

That said, it's incredibly easy to get through it to the structure beneath. Typically, the outside walls are framed in something that's roughly a 2x3 stud and they're very consistently spaced from each other. Find a seam in your siding and you've found a stud. Move over 2' from the seam to the next one (probably 2', they're not usually 16"). Then you can attach to it. Seal the shit out of the penetration to keep water on the correct side.

Even better, don't try to attach anything to it. Make some freestanding structure for your planter.

Amazon "cabinet hinge repair plate" is the best option. They're thin metal plates that go well outside of the damaged area. Less than $20.

I guess that's why Mermaid Man kept him around for so long.

Have somebody else video it. Otherwise, it probably won't get posted.