Metal and I don't get along :-(

When the weight distribution hitch was added, the tray that holds the tanks was re-mounted by the selling dealer.  3 of the 4 bolts that secure it have since broken, leaving a flimsy single bolt holding the entire 80 pounds of tray and 2 full tanks.

What is the proper fastener and method to secure this tray to the steel frame?  Home Depot wasn't any help, they only have sheet metal screws.  When I last tackled the tongue jack, the hardware store guy sold me a lag bolt, which broke again when I tightened it down.

The trailer frame is 2" x 5".  I'm considering a single 6 inch bolt through the entire mess, or trying to find a U-bolt of appropriate dimensions to secure it through the 2 slots on either side.

I'd like to fix this before our next outing Friday 8/24, but am not sure which way to go.