I'm just thinking out loud here. I do not have any experience with this, nor am I a design engineer.
If you positioned the tank up against the front wall, you could use steel straping similar to safety strapping for a water heater. However, this would make your rig extremely tongue heavy for your tow vehicle. If positioned slightly forward of the axle you could run stabilizer bars / bracing between the sidewalls, and fasten to that. Perhaps, even make some storage or shelving in conjunction with the additional bracing.
Just my 2 cents. Hope you can come up with something that will work for you. If I may ask, where did you find the vertical tank? Seems like a great idea to save floor space.
Good luck!
Dave