I don't know how much power a 15WATT Solar panel produces but a 120WATT Solar Panel only produces up to 14.4VDC @ 5 AMPS from its solar controller when in high sun during the day.
Not a whole lot of battery charging power.
My RV Trailer consumes around 1-2AMPS 24/7 with its so called parasitic drain running the sensor panels here and there including the propane detector. We always have a night lite going as well...
If the furnace blower ever kicks ON the blower fan consumes between 6-8AMPS DC current.
Not surprising that a 15WATT solar panel only charging during the high sun portion of the day would keep up with battery usage at night.
Roy Ken