OP I have ask myself this same question, many times. I look at what 2005. 2008 or 2010 units are selling for/asking price now and what they currently would sell for new. I see some units new now would sell for $75K while they are asking $30K for a 2010 for example. That tells me they depreciated roughly $5-6K per year. Although that is a lot, there is also the issue of what a luxury car sells for now and what it would sell for if it were a 2010 model. I know it costs a lot to RV but I do not see the depreciation cost to be that great if you want to have your own bed and control of your environment. Bottom line is if you want the answer to your question, look used prices and compare them to new ones. Sure the depreciation is horrendous if you are only going to own it a year or two.
I also have looked at the 1-2 yr old units versus new. From reading this forum, it seems to me that the common thread is the used units should have most of the "bugs" fixed where the new unit will not. I personally like new for cars as I know every thing about them from day one and drive them until they are pretty well worn out. Rvs may be somewhat different.