The [21RDB] travel trailer is listed by NADA Guides under the manufacturer [Aero Coach].
The  model was part of their Aerolite Series.
Follow these steps to find a base blue book value. Add option and consider the condition along with maintenance to determine a fair value.
Go to [www.nadaguides.com]
Select = [RVS]
Select = [Travel Trailers / 5th Wheels]
Select Manufacturer = [Aero Coach]
Select Year = 
Select Model = [21RDB / Aerolite Series]
Enter Zip Code = 
Option = [Select]
[Get Base Pricing]
Suggested list price = $13659
Low retail = $3900
Average retail = $4680