Here is come things that you pretty much NEED to set up camp, most other things are luxury/bonus items:
Board to level side to side
water hose
sewer hose
4X4 blocks to black under stabilizers and jack (helps to reduce movement)
4 Wheel Chocks
A Hammer (to hammer the chocks tight to the tire to reduce movement)
3ft bubble level
Optional luxury items:
Outdoor carpet/rug
sewer hose stand
pressure reducer valve
hose Y
extra hose and sprayer to clean things off
chairs for under awning
grill/outdoor stove
... and the list can go on and on
After about the first year or after several camping trips, go through the compartments and exterior and interior and remove all of the things that you packed in the trailer, but never use.