While I agree there is no hard fast rule for when to trade. It is possible to keep an RV too long. In some cases use drops off, camping interest fades away as well.
Nevertheless many hold on to their rigs until there is no value left vs. realizing the end is near and selling rig while it still has value.
This theory applies to those who actively use their RV's as well.
There is a tipping point where one is wise to trade there rig in while is still has value vs. keeping it until it is worthless.
I do agree in many cases an RV can be worth a lot more to the owner than it is worth to anyone on the open market.
On the other hand I see a lot of motionless dead RV's lying dormant in back yards and driveways wherever I go.
Dead RV's come in all types,shapes and sizes. At some point their owners held on to them a bit too long and missed the trade in/sell deadline.
Hopefully we will all know when it's the time to turn them in, Unfortunately some of us will not.