Glasses and/or contact lens and spares, medications Rx and OTC, first aid kit, some bottles of fresh water, and a couple days worth of meals for everyone. I also usually carry spare motor oil for the rig, though I've never used it. These are essentials. Check and double check before leaving. Almost everything else can be found down the road if forgotten.
Other items may or may not include "special" items. For example, if you plan to visit Yellowstone NP, know that it can snow anytime, and know that freezing over night temps. are not unusual, thus some cold weather gear is necessary. Know that some areas of the country are rainy, so water proof (not water repellent) jackets are good to have. Swimming clothes if this is on the agenda.
Good luck and safe travels.