Ask away, Leanne - it's only by asking that you'll gain the understanding of all the little tricks. Everyone here will help all they can.
I have always wanted a built-in gas tank - a large 'drum', fitted underneath the vehicle that is refillable with gas from a pump (at a filling station/garage, for instance, though the pumps are available elsewhere - some councils have them, plus factory estates, etc). Maps are available to show you where these filling stations are, plus 'phone apps too. I've never got round to stumping up the money (£500 or so) to have one fitted but they really are the answer, carrying much more gas and in a better position, lower down under the vehicle. These two companies specialise in fitting them so if you're interested, maybe contact them first?
Autogas Leisure 2000 Limited - Refillable Gas Solutions for Motorhomes
or
GAS IT Refillable gas bottles, refillable gas tank, vapour tanks and gas bottles, for caravans, by GAS IT, Alugas - GAS IT Leisure Gas - Catering, Motorhome, Caravan, Farrier and Road Repair Refillable Gas Solutions
The second route is cheaper and has some other advantages/disadvantages, and that's the 'refillable gas bottle'. They are most often supplied by people like
The Gaslow Centre Home Page and from many camping/caravanning outlets. The whole system is set up to be an easy DIY job but many dealers will do the job for you.
Advantages and disadvantages? Under slung tanks carry the weight low down (good) and they are out of the way. They free up the gas locker so it can be used to store other essential 'stuff' - the smaller the 'van, the more important that could be. These tanks can also carry a lot more gas than you could otherwise, so giving you a longer time between fill-ups. The disadvantages are that the whole vehicle has to go to where the gas is dispensed (inconvenient if you're esconced on a great spot and really don't want to move - you
have to). You could carry a small gas bottle to link into the gas line if this was a problem but it all starts to become a hassle then. Also, when you sell the vehicle the tank would normally go with it. You might get a slightly better price for the 'van but nowhere near what the tank cost to be fitted (unless you're lucky!).
The reverse is true with the refillable bottles - they occupy a gas locker, but
could be taken away to be filled if really needed and they can stay with you when you sell the 'van.
Good luck!
Barry