With trades, your modifications don't really add to the value much. They might help get your rig sold. Some say the low retail amount on Nadaguides with no mods added is nearest to the value in the market. With the current supply and demand, you might get more.
Besides
www.rvtrader.com, you should check
www.rvt.com They are similar but different. Be sure to check both Vegas and Axis.
Realize that the dealers often list a pumped up price so it looks like they are generous with trade ins. You might call some dealers and ask for a cash no trade price.