We installed L-track and use Unwin seat lockers with captain's chairs that have integrated seat belts from a crew van. This allows us to move the seats around and configure as needed. Rather than doing "built-ins", all of our gear (bike rack, Dometic dual zone fridge/freezer, water system, toilet, etc.) connect to the L-track as well. It ends up looking pretty spartan, but the configurability has been a godsend as our needs change.
Our typical configuration has the fridge/freezer lengthwise behind the driver's seat plus two captain's chairs for our two girls, and we can swap in up to 3 additional captain's chairs to carry up to 7 people. Behind the two captain's seats we've had many configurations depending on the trip, passengers, and season. We store our bikes inside using 1Up van trays, and the water system typically occupies the right rear quarter of our EB. My wife and I are small, as are my 4- and 5-year old daughters, so we are able to sleep the whole family in the pop-top--but probably not for much longer. We have a Disc-O-Bed that will sleep one adult or two small children (end to end) in the "garage" area, although I'm looking at maybe replacing that with a platform style bed now that our kids are getting older. Currently only the passenger front seat swivels because the driver's seat is powered, but I've been considering changing it to a manual seat so we can swivel it as well.
I don't have a ton of pics uploaded, and the build is always a work in progress because we've been so busy #usingit, but here are some example configurations (ignore the mess):
Let me know if you'd like me to upload some more pics.