First off Grid trip

You will need to use gas for your heating, hot water and fridge. The fridge only runs off your leisure battery when connected to the car with the engine running. If the fridge did run off the battery, it would drain it in no time at all.

Use your battery for lights, water pumps and any 12v sockets.

Check out the threads in Tips For Making Caravanning Cheaper for ideas on solar panels and so on.

Incidentally, you can choose your own user name by visiting My Profile.

https://www.caravanclub.co.uk/club-together/discussions/welcome-hobby-talk/tips-for-making-caravanning-touring-cheaper/how-to-survive-a-non-240v-ehu-site/

Suggest you take a good look through the above. Loads of good advice there.

Generally speaking run your fridge on gas and solar panels to keep your battery charged.

technically the caravan's fridge is powered by the car's alternator when the engine is running - hence recent problems with 'smart' alternators

You need gas for anything that requires heat, this includes the fridge.  No one seems to have asked when you are going.  This time of year I would expect to use around 2kg of gas per day and my solar panels not to keep up with demand.  We spent 17 days in August off grid last year. The batteries were fully charged by 11am and used about 10 kg  of gas.   When makes a huge difference.  

With a freestanding 60w solar panel we’ve toured the Highlands and Western Isles for seven weeks, at least half of which was off grid. Three weeks entirely off grid in Yorkshire and many two week breaks all over the country. A 6kg gas bottle lasts us for about ten days for fridge, hot water, occasional heating and lots of cooking. We’ve changed all our lights to LED’s. The link that DaveCyn has given has loads of information. You might also be interested in the ‘CAMOG’ FB group which is all about off grid stuff. You don’t need to apologise for asking questions - we all started off at the same point. Best wishes for all the lovely memories you’re about to make! 

Apart from using solar to replenish your batteries, no-one has mentioned refillable gas bottles. If you are going to spend significant amounts of time off EHU, then a refillable gas bottle will save you quite a lot compared with the cost of exchange Calor bottles and pay for itself in no time. For a caravan, with limited payload available, the best choice would be "Safefill". A 10KG Safefill bottle weighs the same as a Calorlite (if you can get one!) but hold almost double the amount of gas. The gas will also cost you somewhere around 25% of what Calor charge.  Unless you want to run things like microwave, toaster etc. a lithium battery set-up is overkill. Over the summer months, a 150W solar panel, a decent 130AH deep cycle battery and a refillable gas bottle will see you right.

Make sure you change to LED lights if you have an older van. Also run your fridge efficiently, cool it down before you start (if you can) and don't overload it so that's it doesn't have to work too hard. Don't leave the water heater on, switch off the gas between usage. Our 100w solar panel has seen us through very well even in low light, buy the most efficient one available, some are better than others.

There are other things that help lengthen stays, solar chargers for devices, solar and wind up radios, lamps etc. However we live much the same with or without power just by being a bit more careful with usage.

Therein lies the problem.    The OP has asked about going off grid for 1 week in the future and is presented with a big shopping list.    I absolutely agree that a safefill will save money over the long term.   The last one I bought cost me £190, so you aren’t going to get pay back from1trip.  Personally if I was going off grid as a one off I might trawl the small adds for a 11 or 13kg gas bottle to reduce costs.   I don’t think you need 150W SP in the summer is needed 130Ah battery my 17 nights last summer was achieved with a 90Ah battery.  

It might not be ideal and it is not the  setup I have but a lot can be achieved with a little.  
My Advice for summer use would be.   
A spare gas bottle

Make sure your battery is in good condition. 
Led lights nice to have but not essential.   
That’s about it.  
Don’t let the perfect be the enemy of the good. 

Ps If you are going on a field rather than a site, levelling ramp and blocks might be a good idea.  

Thanks all really appreciate your responses.  looks like an extra bottle of gas, check the lights to make sure that we are using LED (she's an old girl so probably needs them), battery check and possibly a solar panel to keep the battery topped up is in order. Any recommendations for a easy to fit solar set up would be great. I'd rather a portable system than anything fixed to the van. Having googled and looked through the suggested pages there is plenty of choice out there! Thanks again

We bought a 60w folding pannel for our 2nd van and 1st off grid trip. This one.

https://www.sunstore.co.uk/product/12v-solar-battery-charger-60w-briefcase/?gclid=Cj0KCQiAoNWOBhCwARIsAAiHnEjLjVOV4rDY_pelqfRAj9Em62-peXhENjTc8FS1LR6T1fNy3st2sSIaAuNmEALw_wcB

That was 7 night at Tregennis Uchaf near St Davids in August with 6 people. 2 adults , 4 children. 60w was sufficient to charge phones and the inevitable teenager electronics (Nintendo DSs). We didn’t  take the TV IIRC. 
Remember, when off grid, none of your 240v appliances or sockets will work. If you want them to, you will need a suitable inverter to power them and a big battery with a decent solar array.  My advice would be don’t bother and just plan on using 12v stuff only (including suitable 12v to USB outlets for charging stuff.