We have a 2012 back country 18f which is 22 feet. We now have two kids one is 5 the other is 2 months. We are looking at trading ours in but it seems the only size that will work is 33 feet with bunks...
Your trailer will continue to depreciate and regardless you'll not get what you want on trade but that's part of the game. I would not worry too much about a trailer in the 30 ft range in regards to campsites. Get a size that works for your needs and go. As your kids get older and if they still want to camp with mom and dad, 22 ft will be "cozy", maybe too much so.