Using the original battery for the “car needs” was the best bet. I put a small 12 volt charger in just to charge the single battery. Also your pack will go down long before the “car needs” battery will. Both chargers can run at the same time.
I like that idea Greg. I might use that when Treddles is running while I save up for a converter.I have access to lots of smaller sealed 12V batteries which is handy, and considering that your travel batteries would go down faster than your “car needs” battery, I could afford to use a small, light 12V battery for the “car needs” part.