Some times it couldbe the battery. Particals can settle in the bottom of a cell and short the plates. This will cause a voltage drain in the system. If you know the battery is ok you can use a volt meter and remove fuses to pin point the short in your ele.system. A multimeter is very handy I use mine all the time in the shop and around the house.
I hope this helps. If you need more TS details let me know. I have been a heavy equipment mechanic for 30 years and I only own FORDS
:-)