The "sandwich" wall construction seems to give the best "bang for the buck " and although is a bit more work, is lighter and insulated too ... The solid ply sidewalls are heavy and expensive for the "good stuff" (recommended if you go that way ) ...
Reading through the "Vintage" trailer plans , noticed they used a lot of "lesser" materials for construction .... Masonite , cemented canvas coverings, T&G floors ... thrifty old school stuff..
Further thinking produces the idea of a sandwich wall , made with the cheaper wood products like the dreaded luan or even masonite coated with my cheapo waterproofing mix (weight will be a concern )... with EPS insulation ,the blue and pink stuff is much more $... relatively speaking. Small cross section "framing members" (wood), and perhaps a cheap protective covering over all that ..I keep thinking cheap polyester camo net , which is very tough stuff , and can be attached with most anything and painted .. as the vintage boys did ... It does prevent the wood from splitting , as well proven on many ELBs I've used it on over the years ..never a failure ..
Am I thinking TOO Thrifty ????
PS . I think something like this would be best stored under a shed or in a garage , just to insure longevity and low maintenance ..
UPDATE NOTICE: For Eaglesdare's excellent build thread , please go here .. http://tnttt.com/viewtopic.php?t=43647
) I'd set up a big tank and soak all the cut out parts in it for a week ... Wood is like little tubes weakly glued together (like a handful of straws , probably same basic material ) Once you can fill the tubes or at least waterproof (plastic coat) them , water has no place to sink in (soak and rot)... as much ... 
Were you using resin or just glue... ?
Could be "decorative "... 
Cute! 