Teardrops n Tiny Travel Trailers

Was thinking that some have built standies. I remember about 6'6" high.

And many are 12 long or so.

So it you built a standie with interior walls that could act as buttresses/support at least every 12', really you should be able to go as long as you want.
The interior walls become part of the unit.

How would you attach the interior walls to the foam exterior walls?
What do they need to be made of?
The thinner the better to save space.

Could you attach a 1x1 to the exterior wall then 1x4" ply?

Need to think like building a cabinet, not just walls inside an exterior box.
A cabinet is a unit and is very strong, look at the weight we put in it.

Most anything is possible with a little backyard engineering , and a good dose of TLAR ...

I built cabinets as bulkheads at both the nose and just forward of the galley (headboard of the bed). I am very happy with the strength of the build. It's as strong as I would have ever wanted and I have no fears of it "blowing apart" or any of those other unreasonable cautions that have been thrown our way by the skeptics. You can build it as long or as wide or as tall as you would like as long as you keep in mind two principles. 1) composite panels are strong (so use them in your build) and 2) bulkheads (cabinets) provide good anchoring points and lots of strength. There are some inherent difficulties in building with foam but I think they are outweighed by the advantages. If you want my opinions just ask but remember they are just that... opinions.