In nadaguides these seem reasonable, though many will argue nadaguides is at the high end. Compared to a 2980RL, the 3500RL has an additional slide and is several feet longer, thereby a slightly larger (and heavier rig). I also don't know what other features to consider - generator, 2nd A/C, Washer/dryer and the list can be long.
I tried to compare the '06 2980 RL and '06 3500RL with mostly identical features and found about $3000 difference across the board. $19,000 to $34,000 for a 2980RL and $22,000 to $37,000 for the 3500RL. If curious to go to nadaguides.com and click on "RV". There's a whole raft of selections after that to check out.
I've always settled for lower prices when trading anything in, versus selling privately. I think it's because after trading it in, the dealer will charge more for the trade-in and giving a lower price for the trade-in is how they might recoup more money. Besides, you've got to allow for some drop in the offering price since it seems I have never been able to sell things at the original price I ask for even after inflating my original price just slightly to anticipate this. I do okay, but I always have some remorse to get over real quick...
Good luck on the sale. And congrats on the SOB, Robbie. I recall you got it some months ago. Happy RVing to everyone.