Tank locations for a B depend in the chassis selected. On the Express, the biggest space for a black tank under the toilet is just forward of spring mounts for rear axle, curb side. That's somewhat rear of "mid" and not really enough room to put a 75" bed lengthwise behind it.
On extended Econoline, all the big spaces were behind the axle, with another streetside forward of the fuel filler. I know these spaces well, working on my own (unextended) conversion.
Sprinter has a lot more flexibility, spaces foward of axle on both sides for 6-10 galllons of tank, a lot more in the rear, which is why you see most "full bath" conversions on the Sprinter located at the rear of the van. More often, with bath forward of axle, toilet will be on one side, kitchen on other, to use the underfloor spaces on both sides of the chassis.
There is yet more flexibility building B+/C/small A on cutaway chassis because except for tailpipe and fuel filler pipe, all the space outside the framerails is available, and height of space can be much greater than under the van floor, because RV floor is usually at least 2" higher and is often located another 6-10 inches above the framerails. All this space to be traded utilities vs storage bins gives A/C motorhomes more layout flexibility than in van conversions.
You could proably put that bath in a 24-foot Sprinter, foward of that bunkroom configuration, but there may be less room opposite, and you would have a smaller lounge area forward.
To put that bath in a LWB Express, you could maybe flip it and move it to curb side, but to put it forward of that kind of bunkroom, you would probably need a long offset drain from the toilet to a tank located further aft. Or you could do a combined tank for toilet and shower, or maybe better, a cassette toilet.
I don't know enough about under-floor configuration of Promaster and have not yet seen the U.S. version of the Transit. I've seen enough of the Nissan to know that any sort of bathroom might best be done with a cassette toilet, keeping most of the conversion above floor level.