Yeah, the big 4 are...
* Wheel chocks... I use a couple of large aircraft chocks that I obtained from a friend, but any type of good ones will do. Some prefer the scissor-type that go between the tandem tires to really lock them in place.
* Drinking Water hose... 25' will do for most sites, but I have needed more. After a couple years now, I figured out that carrying two 25' hoses work well... and since I spent this past winter in a colder clime than I'd have liked, one of the hoses is now electrically heated. (Bonus item... male & female quick connects to connect to the rig & each other quickly)
* Sewer hose & cradle. The bayonet style fittings are standard, and hoses come in various lengths (10', 15', 20'). Personally, I like the cam-loc style better, but it's more expensive. Like the water hose, I find that 20' is usually long enough, but at times I have needed longer. As for the cradle... they allow the hose to work better (sloped), and many campgrounds require they be used. If the hose is just laying on the ground, it doesn't drain completely.
* Coax cable for cable tv... most campgrounds offer some type of cable tv system, but you'll need your own coax to connect. 25' usually does it, but not always. (Bonus... order one from Amazon with easy-grip twist connectors. You'll appreciate them when trying to connect in the dark, or in the rain, or worse yet... hard to access connection points.)
Other items, you'll find what works for you. I started with a full set of dishes & silverware... now down to a couple of plates, forks, spoons, etc. Same with cookware. As for leveling pads and such, that really depends on the type of stabilization gear you have (manual, electric, hydraulic), and the type of terrain you camp on.
2013 VOLTAGE 3200 (EPIC I & II)
2014 Ford F-450 PLATINUM