I realise some have fridges & water tanks/jerrycans etc. always in them, I'm talking other stuff.
The "list" that's already done & lets you just throw in clothes, fresh food, grog & GO!
I'm yet to buy a trailer, but I know that with all types of camping, the list will require refinement.
This is what I imagine I'll try keeping it stocked with:
Plates, cups, bowls, cutlery
Cooking cutlery, frypans, saucepans, chopping knives & board, etc.
Cooking oil, salt , pepper, tea, coffee, milo, sugar, flour, spices, long-life food(tins,pasta,UHT milk etc)
Detergents-Dishwahing & hand soap etc.
Chux, scourers, tea towels
Alfoil, glad-wrap, clip-seal bags
Insect repellent, mozzie coils & sunscreen
Matches & lighters
Gas Light/electric lighting
Torch & AM/FM small radio with batteries for both
1st Aid kit
Spare rope & tie-down straps
recovery gear specifically for the trailer
Basic tools, cable-ties, electrical & gaffer tape
Pocket Knife
Dust Pan & brush
Shovel & axe
Table & chairs
All bedding (kids & ours)
Toilet paper & toiletries
Quick-erect shower cubicle
Quick-erect shade (maybe)
Toilet stool (I have a wooden toilet seat mounted on a fold-up stool base)
Tarp & piece of shadecloth, spare guy ropes & pegs.