The point I crossed the aisle was approx 1/2 way from rear to back of driver seat, 15 feet. That worked well for me to cross there as that area was determined by everything I built from the left rear moving forward. Those being my woodstove in far rear 5-6ft. Next was where my toilet went because tthe tank is right underneath there. So had plumbing there to do. Figured my bathtub was next moving forward so yup, more lines to run.
For a kitchen sink I used a salon sink just because it was more useful and $10 bucks. That was it from the left side except the feed for the bus water supply. It just came up through the floor where it didn't interfere but it still needed to cross the aisle.
I had no choice but to go both sides as my tank and pump and hot water tank were on that side.
So in all, I had about 6 or 7 runs across the aisle of 1/2" copper.
The crossing was an elevated tunnel I guess about 5 inches, splitting the length of the bus in two. A simple frame dressed with barnboard.
That allowed me to run my rear flooring planks straight back to the door.
Fourteen footers they ended up most about 12-14" wide. Looks nautical to me.
The floor to the front is completely different to the rear but all wood.
Most of my wood was salvaged and old, cut with hand saws and water power saws. Nice effects everywhere I look in the finish and grains.
Sorry, got on a role. Forgot to mention, I had over 200 solder joints to make crossing the floor and connecting everything. Had 2 minor leaks ...so learned a lot about soldering in confined areas. With soldering you let the smoke out, unlike electrical apparatus.