OK, on the bunk length, few will have more than 6'4" for the bunks, some, like the loft on the Cardinal will not have a wall right at both ends, so literally, your feet can hang off the bed. On the twin bed sized ones, you would have to measure actual units, brochures aren't likely to tell you, and trying to measure the floorplans is sketchy because they are not precise on showing wall thicknesses.
As to tank sizes, Grey water (sinks and shower) is normally the limiting one, A navy shower
navy shower can use as little as 3 gallons, more if you shampoo long hair. Dishes and the like you can help by washing in a tub outside. You can save even more tank space by washing you outside.
solar showerskeep your tank clear for other uses, and if needed specialty tents are available.
Black tank size is the toilet contents, and a rough average is a a little less than a quart per flush, so, 4-6 per gallon, yes some will be less, but you want plenty of water in there for effective draining so plan around 4-6 uses per gallon.
