Running 4.5v LED string from 12v leisure battery
These led strings do run the leds in parallel at low current per led, and typically have a resistor to keep the current in check. Simply using the 7805 would be fine, but that is higher than expected for the circuit. 3x AA batteries can be 4.8~5.2V at full charge but quickly drop down to 4.5~4.2V, their normal operating voltage range.
You could use an adjustable regulator to get a lower voltage like 4.4V or you could use a simple 1n400x diode to get a 0.7V forward drop in series with the led pack.
While the voltage and current are low, keep in mind that the 7805 is a linear regulator and while the battery is charging at 14.5 volts you are dropping 9.5V at .17 A or 1.6 Watts as heat. It will get hot. Honestly go the better route and get a cheap USB car charger. Most modern ones have switching regulators and this would be an easy to use, low power draw and low heat solution for a simple led string, compared to a linear regulator.