What size RV
The big depreciations are in the first and possibly second years. After that, it should go down in value more slowly (assuming it is well maintained). Some models will probably hold their value better than others. I'd suggest looking up several years of your model of choice to see how it fares.
www.nadaguides.com or see the blue books at your bank or library.
Be aware that there are several values of interest. There is 'wholesale' which is what a dealer allegedly will pay for the unit in trade. There is 'average retail' which is what a dealer should be offering it for (and perhaps accept a bit less). And there is a value between these two which would be appropriate for a private party to receive for the unit.
So if you pay retail, you probably have eaten the difference between wholesale and the amount you paid or if you sell it yourself, the difference between 'low retail' and the amount you paid. This loss is in addition to any depreciation of the unit.
Also, you may have to pay sales tax, which is another flush of the toilet. Depending on where you register it, you can pay a value based registration fee; flush again.