Agree, you will never get what you put into it. The average person will look at the model, look at the age, look at what the same model & age is being asked for & go from there.
A knowledgeable RVer might like the upgrades & place some value on them but don't expect much.
It is a dilemma that I will be facing in a few years. The trailer will be near 20 years old, had lots of use, had various upgrades, most unseen such as making spring hanger areas much more robust, up market ATS, pure sine wave inverter etc. We won't be getting another so it will be sold almost 'turn key' ready. Everything need already onboard. Tools, spares, chairs, marine grill, the works. Not the usual way a RV is sold.
Even the truck will go, a 3500 CC LB dually. All the buyer will need to do is put personal stuff onboard, load food & go.
US RV base, MD
2007 Alpenlite 34RLR