Hi Gary
What i would try to narrow it down is...
After you have the batteries charged start pulling fuses till you find ones that have a draw.My way since in don't know much about electronics would be to watch for a small spark as you disconnect the fuse or reconnect the fuse.If you have a good tester you probably can test each circuit to see which one has a draw that shouldn't be there.
I know a guy who has a gasser of the same vintage that drains the battery's after a few days.His fix has been to disconnect the battery's.He only uses it to haul water and wood so i suggested putting in a battery disconnect switch.Not a fix but a workable solution for him.
Thomas
90 Chev 3500 c/c 4x4,6.2na,400 auto,4:10 gears.DSG Timing gears,main girdle, isspro tach, pyro,boost,oil and trany temp.Dual Tstats, High volume peninsular pump,on shelf, Custom turbo and intercooler 85%complete. Change of plans for the dually, it's going to get a Cummins. Both trucks are Blue 90 4x4 crews