Like I said, I am a big guy, A normal dinette is too tight for me.
I think that if a king bed is a deal breaker you may be in trouble. A school bus is 96 inches wide on the outside. The walls are approximately 2 inches thick if you don't do anything to the interior walls. That leaves about 92 inches. A king is 76 inches wide. Can you get down a 16 inch space, or 8 if you put it in the middle? How do you make the bed? Get in and out in the middle of the night? A rear engine would also have wheel wells to deal with if you have the bedroom in the back. A front engine flat nose has the engine, that you would have to get around to get to the drivers seat.
A normal size RV bathroom/shower, and aisle space may also be a problem.
There are some coaches that are 102 inches wide, but that only gets you another 6 inches.
Have you visited any RV dealers to see if you can be comfortable in any of the layouts?
I suggest that you find the measurements and do some mock ups with tape/boxes to see if what you "must" have will work in the space given. Sit on your toilet and see how much room you are comfortable with.
It sounds like you aren't planning on full timing, can you "rough it" and make do with some inconveniences when "camping"?
As was mentioned before, 6 people take a lot of luggage and food, and make a lot of waste. You also have musical instruments, is there a bass? Drums? You could still pull a trailer, but then you wouldn't be able to pull a towed.
Just some things to think about.