Most 5th wheels considered suitable for full time are 50amp, have w/d prep a second AC, important if you plan on spending time in hot humid areas. A nice option is a dual temp AC with heat pump, which is nice if you're not paying for electric as in most daily or weekly rates.
If you plan on boondocking you will want a generator and/or solar. We went 5 years without a generator, in those five years I could count on one hand the number of times we weren't on hookups, I can also count the number of times we've used it since then.
Again if boondocking you'll want large holding tanks and fresh water tank. If that is not in your future and hookups are your thing no need for mega tanks. We can go a week with our 60f/50g/50b, it's sponge baths and paper plates.
To me carry capacity extremely important. We have 3000 lbs, before water, propane and to me that is the bare minimum.
These are the things I would look for in a 5th wheel but they are the things that are important to the way we travel. Another poster said, make sure you can use the bed, bath and refrigerator with the slides in, not important to us as only the potty needs to be accessible for us as we seldom travel more than 5 hours/250 miles a day and with a good breakfast there,should no need for more than a potty break for both us and the dog.
I hope my thoughts are helpful.