There are two products that will solve your problem. One (the most inexpensive) is LSL Products "Ultra Trik-L-Start" and the other is Echo Charge (the most expensive). Both product connect between the house and starting batteries. They use the house battery bank to charge the starting battery. When you are plugged in to shore power it uses the converter/charger to charge the starting battery. They are both multi stage and will reduce to a trickle when the battery is fully charged, and both will prevent overcharging. The Trik-L-Start is a 5 amp charger and costs about $50, Echo Charge is 15 amp and runs around $170. Both units are deisgned with diodes to prevent draining of the starting battery to the house bank. Neither will affect the isolater or the alternator. I prefer these solutions as they can be permanently mounted on your RV and no additional 120 volt wiring or plugs are needed. Trik-L-Start can be purchased direct at
www.lslproducts.com or through some RV accessory stores. Echo Charge can also be obtained through RV accessory stores and the manufacturer, but I don't have a link for that. I have not been able to locate a supplier for either at a substantial discount.