We've learned a lot of lessons.
First, the 4″ segmented TiN coated blade for the Fein Multimaster is the secret to this job. See this from Multifit Blades. The technique of scribing the line carefully with the blade oscillating, followed by running the tool slowly back and forth works really well. A little pressure is enough to start cutting a a layer of metal. Eventually, you break through the metal and can follow the starting line, finishing the cut.
The tank's walls seem to be 3mm thick, about 0.13″, which makes it 8 gauge aluminum. The blades say they're good to 11 gauge, so, we're pushing things here. I did finish cutting a pretty nice 21″×21″ opening. I've deburred the edge, so it's not razor-sharp. I have good access to the interior of the tank.
As you can see...
There's a bit of yoga involved, since it's barely 27″ deep. But I can fold up pretty compactly.
I tried some "blind" cuts: sawing up under the edge of the tank where you can't really see what you're doing.
The blind cutting worked out well, but you have to have a good grip on the tool, and a lot of patience. It's not a thing where you can reach a long way inside the tank and hack away hoping for the best. You need to be close and solidly braced.
Originally, I thought I might be able to design a set of rigid tanks that would fill the space. That idea now seems way too complex. The more time I spend, the more I realize how constrained the space is.
The current thought is two tiers of flexible bladders. A piece of plywood can divide the tank into upper and a lower berths. Bladders tend to be less than 10½″ tall when filled, dictating the height of the layers in here. The pointy bit at the bottom-most 3″ can be blocked by a triangular floor. This leaves 24″ to be split into upper and lower berths for tanks.
I think it could be as much as 56 gallons, if I can find the right bladders. And if my estimates are correct. Pragmatically, it looks like the Plastimo 16658 is 115 cm long and 105 cm wide; which is close to the top triangle: 42″ long and 40″ wide at the aft end. This is 30 or so gallons. Underneath it is room for at least a 10 gallon tank; possibly enough space for a Natua FT911121 which is 14 gallons.
Next steps? Measure and mark the location of the shelves to separate upper and lower. Get some plywood and epoxy paint and self-tapping screws and build the shelf. Then. Bladders. Then hoses and hose-clamps.
And once that's done?
The port side of the saloon has to come apart. That's going to be (I hope) easier to get the top of the old tank completely off. And (I hope) easier to order a set of 4 tanks, each 33″×17.5″×11″. Many lessons still to be learned.