Keep it light and tight
I have had my Horizon for a year now. I didn't use is until May of '12. I looked for three months before I bought anything. The main thing for me was quality, light weight and most everything has a bag/carrier.
The trailer has two sides and a main compartment. One side is kitchen and bath the other I call the garage.
Kitchen side: paper towel, TP, lighters, wash cloths, full size pre-moistened full size body towels, deodorizer for porta-potty, soap, mirror, set of 4-plates, bowls, cups, silverware, a few pans, small tea pot for cocoa, larger spoon, egg flipper, sauce stirrer, small cleaning brush, salt, pepper, spices, scissors, fingernail clippers, pen and paper for things we forgot or want to add, and probably a half dozen odds and ends.
Garage Side: Shovel, L.E.D. light for tent, ax, tent stakes, hammer, lantern and propane, lantern hanger, tree for larger propane tanks, hammock, saw, machete, several flash lights, first aid kit, small 12V air compressor, rope, solar panels, saw, small socket set, ponchos, gloves, and a half dozen other things that could get you out of a jam!
Main Compartment: Porta-potty, tee pee for potty, camp fire in a can (live in AZ no real fires this year) lawn chairs, two small tables, fridge, extra water, sometimes extra propane, tent panels.
I take our personal stuff like sleeping bags, pillows, duffle bags in the Jeep. We put our dry food in main compartment also.
Hope this helps.