I think you need to bring in a specialist. 18 batteries in two years, who can afford that, or the aggravation? Take a look in your yellow pages, see if there is a listing for something like "auto-electric specialist". A dealer shop probably isn't qualified for this. You need someone who has an electronic tool that can monitor the current in the battery cable automatically and log it to memory overnight. Then review the log file the next day for anything unusual. There pretty much has to be a time during the night when a big surge of current is occurring. The computer is probably doing something it shouldn't be. But this has to be caught first, so you have proof, otherwise mechanics will just throw up their hands.
One way to prove what I said must be happening is to disconnect the negative battery cable every night. If the battery remains fully charged the next morning, and you can put the battery cable back on, and everything works fine, that pretty much clinches it.
If you were the scientific type, since current in a wire causes a magnetic field, you could probably rig up something like a paperclip balanced on a perch of some kind, that would prove there must be a current occurring during the night, b/c the next morning the paperclip has toppled from its perch.