Image sells. Take some time to "dress up" the TM and take more shots of what living would be like. Put some flowers on the table, afew throw pillows, and take lots of interior pictures (and exterior too). Pretend you are selling a piece of real estate. The impression in the ad will affect how the prospective owner views it in person.
When I sold my last car, I spent a long time cleaning and waxing. I even cleaned the engine bay. I was offered 90% of my asking price so I took the offer on the spot. I thought it would be a tough sell. In fact I got only 2 calls for it. Only one person came to see it and they bought it.
Try to make it look like some one would want to go camping right away. This is not going to guarantee an easy sell, but it may help.
Also, when you say you "upgraded" that makes it sound like the TM is a major compromise. For someone who wants to travel in a 40' 5th wheel, yeah, it is. But that's obviously not who you want to sell to. The TM is great for some one who wants super easy towing, easy storage (especially in the city) and easy set up. I.e., the TM is a major upgrade to a tent trailer.
Those are my thoughts anyway. Some one will want it. Good luck!