Hi Marsha,
We started caravanning earlier this year and were initially overwhelmed by the amount of gear we needed.
However, there are things you MUST have, and things it's NICE to have. From memory, and in no particular order, the following are the MUST haves:
Battery (which you have)
Gas bottle (propane for year-round use or butane for summer only use - 2 gas bottles would be better so you have a spare)
Regulator and spanner for gas bottle
Levelling ramp and chock
Spirit level (so you know when the 'van's level)
Electric hook-up lead (25m)
Toilet chemicals (pink for the flush, blue for the cassette)
Aqua Roll (or equivalent)
Wastemaster (or equivalent)
Wood (for levelling)
Corner steady winder (if you don't have one)
Submersible water pump (if you don't have one)
Insurance (if your 'van's in storage you'll probably require this - check the small print of any policies)
Wheel clamp and hitch lock (your insurance company will probably require these)
A noseweight gauge is usefull but can be improvised with bathroom scales and a broom handle
Towing mirrors
And that, I think, is all you'll really need to buy before you go! Anything else, like melamine plates etc, you can get over time. Just take everything else (cooking stuff, bedding etc) from home at first - you can buy specific stuff for the 'van as time goes on.
Incidentally the best place we found for kitting out with the essentials was the following website:
http://www.thecaravanshop.co.uk/
They are very helpful (refunding the purchase price AND POSTAGE of a wheel clamp I ordered in the wrong size) and were by far the cheapest source I found for caravan equipment.
Have a great time!