Hi Peter,
There were a few threads on this very subject recently 
Check these out for some more ideas:
It’s entirely possible that more batteries are the cheaper solution?
Also, you could add this to the list:
- A 12v Mavic Pro car charger
It’ll still drain the car battery eventually but it must be a more efficient method than an invertor.
The spare leisure battery might not be a bad idea, you don’t have to remove it from the car to use it, leave it in the boot or footwell?