Teardrops n Tiny Travel Trailers

I have been reading somewhere saying thicker foam is better... But also confused about how the strength does not come from the foam...

My plan is to use no wood and all foam, no shelf, no roof rack.

Which is the thickness I should use, 1.5 is perfect but I am afraid it's not strong enough.

The secret of foam is that twice as thick is many times more than twice as strong … So the thicker the better ( within reason ) , more insulation too … And then some foams are stronger . On our trailer , we used beaded foam , which is not quote as strong as extruded foam ( blue or pink) , so we made our walls 3.5” thick …
For “stressed “ areas , you can always add foam doublers to create more strength in that area.

On my foamie I used 2" thick walls.
My unit is 11' x 5' so quite a big structure. I used some supporting framework inside as I felt it was needed for such a big space.
MY suggestion would be to use 2" XPS (pink) foam as they are quite rigid and provide a fair amount of strength.

My 4x8 is of 1.5" blue foam.
Its holding up well.

Thank you everyone, I went down staring at my trailer for a while and finally decided that 1.5 is way too thin... Tomorrow I will browse around home depot to decide which ones to pick up and will update... I'll probably start a journal next week.