I've built structural panels with foam cores and plywood skins, and have looked at SIPs for buildings, but had not seen manufactured panels that might be appropriate for small trailers.
https://kerfkore.com/lightweight-panels ... -foamkore/ thickness 3/4" up to 3", several skin material options as well as eps or xps, and up to 5' x 12'. Offhand seems more suitable to rectilinear designs but there might be hybrids that use this for walls and a more traditional material for the roof and ends.