A Porcelain/China Bowl Toilet. Also "Right Height" if available.
Large Oven.
Double pane windows.
Screen Door. (I see many units without these days.)
Black Tank Flush. (I think most have these now)
The electronics (tv, dvd etc) are usually inexpensive, low quality stuff. You are better off purchasing something decent yourself.
Buy something used. Try it out for awhile and see what you're missing, wanting or don't really need. You might find you don't even like the whole rv thing. After a season, you will have a better idea of your wants and needs. The odds of purchasing the perfect trailer the first time are extremely low.