You need to approach this in an ordered way.
You are getting a replacement battery - Good.
You need to verify that your charger is in working order, if it is not it will ruin the new battery. You should purchase a cheap multimeter, if you are visiting Halfords you will be able to buy one there. Check the reading at the connectors to the battery without the battery connected but with the charger on. If you get a reading of less than 15v, connect the battery and check again. An initial reading over 15v or a reading over 14. 5v when connected to the battery would indicate a problem with the charger.
If you have a faulty charger then you are going to need some professional help to sort out your electrics.
If you still smell something petrol like then follow the fault finding options suggested earlier by happynomad:
And get a working carbon monoxide detector .
The results of these tests may require you to take action which you cannot afford, however the advice you are being given here is for your own safety. Gas and electrical problems are dangerous and potentially fatal.
Better switching things off than exposing yourself to that risk.
Would you post where you are situated? May be there is someone who can help nearby, or failing that a member of the local Caravan and Motorhome Club centre could ask around their local members for someone competent who could come and help you out.
If it's as per your profile here is the CMC centre Facebook group.
https://www. facebook. com/groups/gloscentre/