I tried to reply to you to no avail, I wanted to give my input to your problem as I have been in the ag/auto/aviation repair business for over thirty years,IDO would not let me in even as a new member,so am hoping you get this.

I have a couple of thoughts as to what your problem is,one,have your motors alternator checked for A/C output,shouldnt be over two milliamps at any rpm's,this is AC voltage that will cook a DC battery or batteries.

second,have your battery tender checked for the same thing as the diode may be defective,seen it more than once on new stuff.

third,is there a drain on your system that is depleting the batteries?even with a onboard charger,it may not keep up with the drain.

fourth,are all grounds and positive connections clean and doing their job?

have you checked all loads and grounds to their potential??IE,use an old high power spotlite to load them with at their furtherest point,(max load here)

there is absolutely no reason to go through that many batteries,if you are a mille lacs website member,pm me,I will do what I can to help,Iowa boy,