Thanks for the suggestions!
In reply to Norm/mezmo, I discussed the Singpanels again with the company and the owner would be very interested in making something for a teardrop but we both realized that it it wouldn't be practical to do so - even if he did it at a very reduced cost (just for fun and making no money, maybe even loosing some), it would still cost more than sides made of marine plywood. I could have taken him up on this but even if I were spend the extra amount for this experiment, it would present more project risk of getting the design exactly right the first time for my first teardrop build - if I were to make a mistake along the way there would be a repeat cost/effort/drive for both of us to get a new panel that would be more difficult than getting another sheet of plywood.
We also discussed how a rot-resistant puncture-resistant floorboard in any RV is desirable because if it ever rots one must remove the rest of the cabin to get to it to replace it (removing the cabin might be intact or in pieces, depending on its design). He could make a panel with aluminum skin on both sides and filled with either polystyrene or urethane foam and the wood grid he uses to make it very stiff, light weight and durable. Such a panel would run a few hundred bucks. I could consider it but I already have my aluminum honeycomb sheets in the garage for mine.
In reply to Forrest747, thanks for the observations. You apparently made a few more clicks and spent some time to open the pictures to see that detail. The idea of the 2x4 cleat at the bottom of the side wall is to have a raised false floor for storage leaving a cavity 3.5 inches deep (I've seen this on the Silver Shadow). Otherwise a 2x2 cleat would be sufficient. had put the foam insulation in the galley just to reduce the weight, but you make a good point that I don't need insulation there so I may just keep it simple and avoid the extra work and leave it with more "meat" in the walls to attach cabinets or hardware. I'm not too concerned about the insulation because in most of the areas I'd use it, it doesn't get too hot here in the PNW, and even wood would be more than a thin nylon tent has given me for insulation over my last few decades of camping.
I had seen Becky's build, and hadn't found Zack's until you pointed me to it (I had spend many hours looking through this and hadn't previously stumbled into it - goes to show how big this forum is!). I like the idea of MDO plywood that he used and have found that I can get 4x10 sheets nearby, and maybe even 5x10. The interesting twist is that the plywood wholesaler that most likely has it is about 2 miles from my house but I'd end up ordering it from a lumberyard about 8-10 miles away.
Then last thing I'm thinking through before buying the plywood is figuring out how to have more interior height than a simple 4' wide sheet of plywood would allow, given the way I'd loose vertical height with the way I'll likely attach the wall to the side of the frame and the storage under the false floor. I'll either get 5x10 sheets, splice some together, or try something like Zack's wide roof ridge (that is inspiring, but I'd go without the convertible roof).
Meanwhile, I have been playing email/phone tag with the welder to order the trailer frame and I hope to conclude that in a day or so that he can start fabrication and get the axle if he doesn't have it in stock (he has some torsion axles in stock but not sure if they are the size I need).
Steve