Battery drain

Reference... Itasca, Ford chassis: My converter has recently began running much more in order to keep the house batteries charged. In trying to find the problem, it appears that there is a drain causing the batteries to lose up to 15% of charge in an overnight period. That's without any 12v system on. Question: How do I go about locating the apparent cause of the drain? Any thoughts, ideas, comments will be appreciated./ Larry