This idea came from my parents. We went camping for two weeks most years when I was a child. This was a long time to be away from home so careful planning was essential.
We had a big list of everything we needed to take with us to make sure we didn't drive 400 miles to the campsite and find that we'd forgotton to bring any spoons.
Every frequent camper has a different list as they build up a collection of the stuff they need over the years.
But here's my list as a starting point, as PDF and as an Excel spreadsheet with tickboxes for easy editing. Thanks to Yinan for the idea of webifying it!
Just a few notes:
Sleeping bag - VERY IMPORTANT: go out and buy the biggest, thickest, double (two person) sleeping bag you can find!
I keep most items in a giant, tough plastic chest. Mine is made by Contico, I don't think you can buy the exact one any more but it's a bit like this:
I use a mains inverter to get mains electricity from the car battery. To make this work, I have to keep the inverter in the car and run a mains extension lead to the tend. If you run 12v into the tent and put the inverter there, it doesn't work (due to voltage drop along the cable at low voltages). HOWEVER, my lights work of 12v, so I need two extension leads - mains and 12v.
Lights - I use a 2 x 8 Watt light, with a bit of wire coat hanger to attach it to the roof.
Heater - I have one that sits on my gas cylinder. I wouldn't bother with the little cartridge types as the gas works out very expensive. Find these on google as "tank top heater". Make sure it has the right connection for your gas cylinder!
Coolbox - I use an electric one powered from the car battery. Ok for light use, but you can't leave it on day and night or it will flatten the car battery. You can get gas powered ones, which are great, but expensive and bulky.
Kitchenette - this is a home made wooden frame that fits perfectly in my trailer. It has a couple of removable shelves and is ideal for storing things. You can buy them ready made (google "camping kitchenette") but they're expensive and not as good.
Big Table - I got a cheap plastic patio table, sawed it in half, and got some bolts to join the two halves together again. This is much bigger and cheaper than any folding camping table you can buy.