You lucked out with rubbing on the axles not wearing a hole in the tank(s) and then having to replace them.
If I were you, I'd run 2x2 angle iron under each side of the outside edge of the tank and drill the cross members and bolt it on through them with bolts, nuts and lockwashers with one inverted angle down the centerline of the tank (front to back).
You aren't the only victim. Many threads on the Forest river forum about tanks actually dropping out on the highway. Can you imagine driving behind an RV on the freeway and suddenly have a water tank appear right in front of you (following behind). Not good
Like I stated earlier, the manufacturer don't care, the units ship with empty tanks so there is no weight involved and plumbers straps work fine... when empty
There was even a couple threads where the owners had covered under belly's and the tanks were bowing the underbelly covering out...
Amazing how cheap the construction is.... and expensive the retail price is. Somebody is making a lot of money.