We did not want to put them on the bumper for too many reasons.
What we did is used our dinette area to install the bikes.
When you remove the seats we flipped over the wood seats. Installed the fork
mount into the wood seats. Then in the rear put to small pipe 1/2 clamps. Secured these with bolts to the wood using T-nuts. Then put the forks in the front clamp. Used cords to hold the rear down with the clamps.
Then repeated the process 4 times, but flipping the direction of each bike.
Then made sure the bolts were below the wood surface so when the seats were flipped over there was not issue with the dinette seats.