My 1.9 gives me circa 600 miles from a tank. 350-400 of them will be from full to half on gauge, so it does indeed drop quicker for the second half.
Ditto.
Before you ply line and insulate take a good long think about what you want to install, leisure battery / split charger upgraded stereo , lights etc. putting in the cables now will save you a lot of bother later on.
This. I’d add insulation to the list too.
And have a hard (i.e. not carpeted floor), makes cleaning much, much easier because you can dry brush it (or even mop it if it’s really bad). You can always use big cheap doormats to cover some of it to keep your tootsies warm 🙂
If you’re near Ciren or Swindon you can have a look at mine for ideas if you like. It’s practical rather than bling, but still is a comfy camper/bike bus.