As I recall from my Casita
days, black tank in a Casita
is above the floor, under the toilet. That valve is in the line of fire, I am kind of surprised it hasn't been knocked off.
I would think a 5 inch high black tank would be problematic, think poop pyramid; especially since you will be boondocking
. Some dry campers like myself use the toilet for #1 only. Instead, figure the black tank will be more like the distance from the bottom of your drain valve in the picture to the trailer shell. Guessing more like 10 inches.
You may be better off with a composting or cassette toilet.