The amateur shopper, defined as someone who does not buy and sell vehicles for a living, places far too much validity to any "Book" value. The professional knows value is market driven and follows the law of supply and demand. They also recognize that the older the unit, the more value is determined almost 100% by condition.
When buying a twenty year old RV,you will find examples that would be a mistake to buy for 10% of guide value and others that would be well worth 150% of guide. At the 20 year point in any RV's life-cycle you are simply looking for visual appeal, usability of purpose, and projected reliability. Overall condition is the only criteria at that point.
