There will always be what ifs to deal with… demand, location, season, condition…
You still need a reasonable depreciation plan to work from, a mark to meet or beat unless you don’t care about how much your loss will be or believe in just winging it all the time…
I don’t use the NADA or KBB either… this is a schedule I used to buy and sell RV’s when I was younger that seemed to serve me well, I don’t know where it come from or who to credit for it, I just found it to be a reasonable point to work from on any type of RV or in any economy… it may not be of value to anyone else… everyone sees value in a different way…
It saves time when buying to… if the asking price is too much out of line, I can quickly assume the seller is underwater and will most likely not be able to sell it to me at a price I am willing to pay… so I just move on…