I doubt if either the pantry or bath wall would be strong enough for a hammock.
The simplest solution would be to build a frame out of 80/20. I'm thinking something like a crate - two 4-piece platforms and a leg/riser at each corner connecting them. Probably all out of 1" x 2" pieces - it's strong stuff. 12 pieces total. You could use plywood or, if you want to be fancy, honeycomb panels for the mattress platforms. You might want one 1x1 mid brace for each platform. If you really got into it, then you could do multiple cross braces and then set the platforms into the frames so that you end up with a 1" lip all around to hold a mattress pad in place. You'd have to measure carefully to account for the angled outside wall, radiused corners, etc. Set the frame across the dinette seats, fasten in a few good points to keep it from moving during transport, and you're done - though I'd probably want to bolt it through the seat bottoms to make sure that it doesn't tip when climbing into the top bunk. You might build a ladder out of 1" x 1" extrusion and attach it to the face of the frame. Their standard corner connectors would probably provide plenty of rigidity, but if you want to be super safe, then they have diagonal corner braces too that would guarantee that it doesn't rack. The whole thing would make for a nice storage platform even if you don't need bunks.
The only drawbacks would be cost, since there would be a lot of connectors, and maybe weight - those extrusions are pretty stout. But I've built stuff out of it before and it's super easy to work with and incredibly sturdy. You can cut it with a miter saw and it all goes together like an erector set. Slam dunk. Go for it.
edit - like so: