There is a hunting community. The area is a well known spot for samba deer. Plenty of experienced hunters take people out there to hunt as well. Experienced high country muster who assisted the police with the initial search for Russell and Carol said you would come across deer remains every 200-300 metres.
It has been stated that the trailer was Russell's* and was listed on the missing inventory list. It's been assumed it was taken to transport the bodies, and those bodies being inside the sleeping bags.
(*Unless Motomike has confirmed it was not Russell's)