Depreciation is also a variable that depends on outside forces that we all have very little control over. If, for reasons beyond our control, the price of fuel doubles in the next year or so, the sales value of large Class A RV's will drop a huge amount. If the price of fuel drops in half, then more RV's will be sold and the sales price of the good ones will increase.
Also, many people, including me, think that the stock market is higher than where it should be. If there is a major downward correction in the stock market, the sales values of RV's will plummet. If the company that built your RV goes under, that can also affect the sales value of your RV.
Basically, buying an RV is a gamble. There are too many things that can affect the depreciated value, that are beyond your control, to be of any use in trying to calculate the future value of your rig.
The best bet is to never "invest" any more in an RV than you can afford to lose. If, at some point in the future, you sell or trade your RV and receive any cash value for it, just consider yourself lucky.