Small (5W) solar panel does work to keep batteries okay

If your batteries are in good shape, you can expect them to still be at a near full charge after 6 months as long as you remove any parasitic draw from them. Simply remove the ground cable.
I have a '57 Ford in storage and it only gets started twice a year and driven only once a year. The battery stays charged and even with the mechanical fuel pump and carb it takes 15 to 30 seconds for gas to get to the carburetor before it starts.