We had 2008 and 2011 3400's. The flooring is around 1/2 inch thick. I screwed some "D" rings to the floor for straps, and made sure the screws were no more than 1/2 inch long, or depending on how thick whatever device you are screwing down, the screws will go no deeper than the 1/2 inch thickness of the floor.
If you can figure out where the framing and supports are underneath, and keep the screws over them, then the tanks would be out of danger.
Terry and Patsy
Vietnam Veteran, US Navy
2015 GMC Sierra 4X4 3500 SRW