I'm not sure where you can get a 'good' idea of value.
However, you can start with the nada value
https://www.nadaguides.com/RVs
and then go to
www.rvtrader.com to see what rigs are selling for in your area.
Keep in mind that dealers will give you a fraction of those values. You'll make more cash by selling it yourself. Depending on your state, the trade in value may decrease the taxes you pay significantly so you'll have to do the math.