You can go to the NADA guides:
http://www.nadaguides.com/RVs and do some comparisons of 3,4 or 5 year old trailers to see what the average retail is for each model. The original MSRP is listed with each model, so you can easily compute a "percentage of loss" for each trailer. That will give you some idea of how much each depreciates in a given period of time. Of course, the "false depreciation" factor will come into play when you consider that virtually NO trailer is ever sold for MSRP, so depending on how much discount you get, the percentage of depreciation will differ from those calculations.
Honestly, for me, I'd be more inclined to get a trailer that I like, with a floorplan that works for me at a price I can afford and I'd not be overly concerned with how much it will be worth 5 years from now. Things change so dramatically (think back to the gas shortage, the energy crisis, the downsizing of RV's). Who knows if we'll even be allowed to tow on public highways in 5 years????? Yes, I know that's an extreme statement, but just because a trailer is a "good investment" today doesn't mean anyone will want to buy it in 5 years, which makes it a complete loss.....
Any time you think recreational equipment, whether it's Ping golf clubs, a Chris Craft "yachet", a snowmobile or a ATV, chances are it'll cost you much more in dollars than you'll ever get out of it in "recreation". We buy them because we want them, not because it's a good investment......
As for which will last you the 5 years? Remember that the least expensive RV that Keystone builds, the Summerland, qualifies for the same 12 year financing that banks lend on the Montana and MHC, so from a "loss perspective" it looks like the banks expect the cheapest to last as long as the most expensive brands from Keystone, otherwise they'd only offer 36 month financing on them.....