Contact the appropriate regional office of BLM's Law Enforcement Desk. Tell them you want to report some long term unoccupied RV's being stored in an LTVA and that the camp host is not responsive. The fact that the host doesn't know who owns one of those RV's indicates they are not doing their job. If it gets the host in trouble, so be it.
If the owners did in fact obtain permission from BLM to be absent in excess of 12 days, the appropriate regional office will have that on file. If they just upped and went away or the host is kinda turning a blind eye, let BLM's enforcement arm handle it.