Green’s Dictionary of Slang

(US) a person, an individual; usu. in (ironic) phr. happy camper.

1974		J. Lahr Hot to Trot 127: You’ll give up. You’ll be a good camper about it.
1983		C. Heath A-Team 2 (1984) 107: Mornin’, campers!
1990		R. Doyle Snapper 35: Hi-dee-hi, campers, Jimmy Jr greeted them all when he came into the kitchen.
1993		M. Dibdin Dying of the Light 20: ‘Good afternoon, campers!’ he called jovially.
2012		J. Fagan Panopticon (2013) 189: Morning campers, it’s going to be a good one!