3/4" is fine, I had to put a converter in line to get that big as my outlet is only 15mm. I'd thought about it for a while, interesting that a converter has come up with the same idea. Yes that's the webasto thermatop I think it's called, quite a few on here have one and it's standard fit in some markets. I presume eberspacher do a similar one. My water heater works off 12v and 240 so I can use it straight from battery, but it will obviously be a lot less powerful than yours, esp with lithium. Is it LiPoFe or whatever it's called, and what charging setup do you have?
It's a great setup, even more so that you've got it to 4wd. I'm all for alternative layouts to the traditional side one, especially if you're rarely going to use the downstairs beds. I can see now how you got it up to that price if it's 4motion with all the options ticked, but I'd need to be really confident I was going to keep it for a long time so that depreciation didn't massacre me come sale time.