So for what all that's worth I would tend to think I may be able to get quite a bit of life out of properly painted exterior grade plywood or siding with a lot of attention given to the preparation and application of a good paint. If I thought I could skin the roof with EPDM or aluminum (which ever would be cheapest) and paint the sidewalls and get 15 years out of it parked outside year round I would go that route.
Does that seem reasonable to hope for? After all I don't want it to last too long because I will probably get some more ideas for another build at some point and I may need an excuse!
Without,
It sounds like you're on the right track!
Your experience with using luan for the dog crate and doing a basic slap it together and throwing a little paint on it shows why most exterior projects start to leak and fail.
Pretty much any exterior plywood product, or for that matter an interior plywood product can be used for a trailer, the key is in understanding that the wood must be 100% sealed/protected from the elements.
Once you understand the importance of that theory, it is much easier to seal/protect the wood in a more logical order than the build a box, then try to seal it approach.
Skinning the roof, and or walls will work just as well, with a bit less maintenance, but again, in order to prevent leaks, you must first understand what causes them, and plan your build accordingly.
15 years is not unreasonable to expect from a correctly built and sealed trailer, however, it is important to note that an annual inspection of the exterior of the trailer is required if kept indoors, and semi annually if left out in the elements.
Because you just built it, or bought it, and much attention was paid to sealing it correctly, doesn't mean you can just forget it for years at a time.
Rob
Waiting for "someday" will leave you on your deathbed wondering why you didn't just rearrange your priorities and enjoy the time you had, instead of waiting for a "better" time to come along...