Coug, You didn't mention what type of rig you are thinking about, 5er, TT or coach.
Our short list of "must haves" included:
Dual pane windows which offer more insulation from the cold and noise.
Leather seating, holds less dust and dirt, easier to clean.
Extra batteries, as many as I could fit for dry camping.
Solar Panels, to recharge those batteries.
Obviously extra batteries and solar panels aren't anything you can order from a manufacturer.
Electric steps are usually a standard with most motorhomes don't know if they are options for trailers or not.
I bought the side view cameras but just can't get my brain to remember it's there to use, just used to using my mirrors, however I do like the built in LED turn signals in the camera pods though.
We love our backup camera with sound so that you can actually hear the wife shouting at you when you back up (hmm maybe this isn't a good thing...)
Not having your knees under your chin while on the throne, be sure to sit on the toilet when looking at RVs....