One (heavy wire) goes to the main battery terminal. One (heavy wire) goes 
to the auxiliary. One goes to ground. The switching signal (smaller wire) 
is usually wired to the alternator warning light.

If you don't have a service manual with camper wiring diagram, you can 
borrow mine (I'm across the parking lot :)

+ Morris Arthur                                       morris_arthur@unc.edu
+ Center for Multiphase Research                 http://cmr.sph.unc.edu/CMR
+ Environmental Sciences and Engineering, UNC   http://www.sph.unc.edu/envr