if the batteries are the same Voltage, you can wire multiple batteries in paralell and use a Voltage regulator and some electrolytic caps to keep the DC voltage regulated.
i favor the LM78xx series, in your case, the 7805. u may also be able to use a Zener diode to regulate the voltage if the current specs are observed.
Those big lantern batteries are pretty decent with available power, your best bet may be to use Bigger single cell batteries, like C or D batteries in series banks of 4 to give you about 6 volts, and then paralell banks of those.
im not sure hoe big this vehicle is, but if the weight isn't too big of a deal, larger cells would be the way to go.
also, the 7805 regulator if you draw over 500mA, you want a heatsink to keep it cool if you draw the full 1.5A
hope that helps!