You don't need a ton of clearance between the axle tube and the tank bottom, axle articulation (up and down) will be only a couple inches at most. You do need to remove the cheap straps and install substantial angle steel to secure the tanks from movement because at some point without being secure from movement your plumbing lines to them will fail and the tanks themselves will fail.
Typical cost cutting build, using cheap straps. I sort of remember Forest River advising owners not to travel with full tanks but to fill them when arriving at a campground and then emptying them when leaving. That makes no sense to me at all other than mitigating the liability of Forest River for sunbtandard construction.