My camper originally had big rubber stops, about 2" long.
At the time when I bought the camper the stops have been already loose on 2 cheap screws so I replaced the 2" rubber with 1/2" of ape wood (very hard wood) and having flatbed at the time that worked perfectly bringing the camper to front wall with minimal gap.
Now new F350 has a bed with top bend inwards, so I have to put 2x in front of the bed, or camper will not hit the bed railing. Still with no visibility of stops when I load the camper, I think tapered wood is safer than rubber stop, that proved easy to bend on any mishap.