I started modifying our TT after it's second trip out way back in 2002..  It was still under warranty too!
First thing I did was remove the solid bathroom door and replaced with an according door.
Started adding shelves to the cabinets and closets.  Built in a sliding pantry and I've removed the dinette setup and replaced the stereo and still need/want to do other changes to it..  Just need to do it! :)
The thing with these RV's is that everything you see inside was added, so it can be removed or modified to some extent.  There will be some things you just can't switch around without some major work of course, but anything can be done with enough time and $$ :)
Getting a dealer to do that work when you buy from them, is probably not going to happen..  They just want to move rigs.. They aren't usually in the business of customizing them..
Anyway, depending on how handy you are with tools and your imagination, you can do a lot..  Just some examples of what I've done over the years..
Mitch



