Keithbelson

Hi, I have a problem with the battery draining. Mostly it's good but sometime the battery goes dead, sometimes overnight and sometimes if left for a few days. When the battery is dead I cannot open the tailgate and the battery is in the back. One time I came back from Bardsey Island to find that problem. I did not have access to electricity. How do I check for draining when with the tailgate open the dome light is on?