Battery charge retention in the motorhome

For winter storage I bought a 30 watt solar panel that can produce a maximum of 1.5 amps of electricity. Under clear skies and very good sunshine, I measured the electricity production behind and in front of the windshield of my motorhome at the end of August in southern Finland.
The measured values ​​behind the windshield in bright sunshine are approximately 0.93 amps, which corresponds to 12 watts of power. I was able to measure 1.23 A in front of the windshield, which corresponds to 16.1 watts of power.
I would like to use the 30 watt panel to maintain charge of the starter battery in order to charge it using a cable with a plug that fits into the cigarette lighter socket.
In the meantime I bought another solar panel. It is a 50 watt panel that I want to use to charge the two on-board batteries that are located under the driver and passenger seats. I connect this solar panel with a cable that has two alligator clips to an on-board battery under the passenger seat. Since the two on-board batteries are connected to each other, the other on-board battery under the construction driver's seat is also charged.
I only took further measurements behind the windshield because I place the solar panels this way to protect the panels from rain and snowfall. My alcove motorhome has a certain amount of protection because of the structure that covers the windshield.
What is the performance of the two panels when it is cloudy or has little sunshine? In heavy cloud cover, the 30 watt panel delivers 0.2 to 0.33 ampères, which corresponds to 2.5 to 3.9 watts.
Here are the comparison values of the two panels listed for different levels of sunlight that I measured.If the sun comes more out of the clouds, performance increases. Here are the measured values of the two panels in comparison.
In this calculation table I created, because of the different weather in winter, I made a maximum and a minimum calculation and included a daily discharge of 2A. This leads me to the assumption that trickle charge should work. The lowest level of solar radiation is reached at the end of December to the beginning of January.
From January onwards there will be more sunshine again. At the beginning of June I will see how well the batteries survived the winter and then make another video. In the meantime, thank you for your interest and see you in the next video.