I don't have "accessories" in my camper. Everything in my camper is a necessity. After trading our last camper (3 years years ago) and cleaning it out, we decided we'd get rid of the "accessories" and keep the "necessities". Things have been less "pack-rat-ish" ever since.
What's important for one person may be totally useless for someone else. I don't ever hook up to city water. I always fill my fresh water tank and then turn the spigot off at the campground. So for someone to say the latest technological advanced pressure regulator is an accessory "I" need... well... sorry... I do not need a pressure regulator ... at all, since I never hook up directly to city water.
So, really, what one person calls an "accessory", someone else may call it "necessity", some else may call it ... simply, not needed. It really all depends upon the type of camping you do what you really need, want and think you need that determines what you need.
As far as basics, I think a good garden hose, a good sewer hose, enough electric cord to hook up to electric (here again ... this is subjective? How far will you actually be from the plug? But you need to think in terms of the extreme case, so you have it if happen to camp at that extreme end.). The other items you need, actually every one does, is something to level the camper with. Be that boards, Lynx blocks, but something under the tires to level side-to-side. After this, absolutely every thing else is personal choice based on your comfort level.