If you're not pulling a 5th wheel or in a MH get a tailgate pad bike carrier. Very secure, won't damage your truck.
Otherwise, keep the bikes inside trailer or MH.
Only other suggestion would be to have a welding shop add some truss/gusset supports to the bumper. The small spot welds currently holding it on won't support much weight. And then get a platform style bike carrier. Bikes won't move around or bounce around with this style of carrier.
DAKINE Pick-Up Pad - Truck Bed Mounts | Backcountry.com