The Central American nation secured shock victories over Uruguay and Italy to seal their progress from Group D with a game to spare and they only need a point against England on Tuesday to win the group.
Arsenal striker Campbell says he has no preference as to who they face in the knockout stages and believes Jorge Luis Pinto's side have nothing to fear.
"We are not thinking about who we get in the last-16," he said.
"It does not matter to us. We have been in a group with three champions of the world and we have shown that we can beat anyone.
"We always believed we could do this. Football is not decided by the shirt you are wearing. With work and effort, you can win against anyone.
"We are very satisfied, very happy with what we have achieved but we will keep working to do more.
"This is unforgettable. We are part of history. Nobody would have put a single dollar on this team qualifying but there you go.
"It can change football in Costa Rica. These are the best days of my career."
The winners of Group D will face the runners-up in Group C, with Colombia, Ivory Coast, Japan and South Korea competing in that pool.