We have trailer vision and seems to work well. My mechanic did fit a switch to turn off the horse camera as apparently it didn't have one and may have drawn off the battery if I left the isolator on for any reason. The human viewing monitor is remove able so as long as you remember to do so,can't be stolen. http://www.trailervision.co.uk/index.php?main_page=product_info&products_id=180
I actually brought a reconditioned one from them,at quite a lot less to their new ones.
Trailer vision do a dual sat nav/camera so 95% time on sat nav - I rate them, and the technical help and customer service (unless changed in the last couple of years). As mentioned, make sure it is isolatable from the main battery, either through ignition shut off , or separate switch (I had a redundant switch on the dash, so got mine plugged to that)