Not familar with that convertor/charger but based on its age, it is probably a dumb charger. It is typical for the charger to ONLY charge the house batteries. The options are to use the battery merge switch (which should not be left in the merge position all the time or you will likely cook your starting batteries) or get a trickle charger. A neat little trickle charger is the Trik-L-Start
http://www.lslproducts.com/TLSPage.html